How to deal with a stubborn user that constantly makes mistakes, but refuses to accept it's their fault

Question

I'm making a desktop app for my best friend's family restaurant (no, I'm not doing it for free).

Background info:

My friend told me he tried with several apps he found on the Internet; in fact, he claims they have worked fine for him ("by fine," meaning no bugs or crashes, the new meals are added, the bills are calculated with the right amount, etc.), but are very graphical unattractive at best, hideous and trying to leave you blind at worst (who thinks mixing yellow and red is a good idea for a GUI?), and lacking in features we wants / having features he doesn't need, so he figured he needed the app made for him.

Example: Someone and his/her buddies go to have their meal. He gives them the bill and cue some jerk in the group with a sattisfied sneer on his/her face: "Oh, didn't we mention everyone is paying for their own stuff?", while the others snicker. They have previously said X friend was paying for it all, and this... gaslighting starts about how they hadn't said that, how my friend has a "bad memory and shouldn't be a waiter", etc then he has to go back to the computer and do it over (and then the guys whine he took too much time to bill them, and start demanding a "discount" for the long wait.) This happens all the time. Resourcing to ask them about who is paying what at the very beginning doesn't work either; they lie.

The feature he urgently wants is to quickly separate bills (now, he deletes the order and does several bills for each jerk, which takes its time), to stop these bully people for causing trouble and complaining for his tardiness.

Now, he's a role model user. Excellent, in fact. If he has a question, he asks. If he doesn't like something, he says it. He didn't understand why stuff is rendered "inactive" rather than be deleted; I explained it to him and: "Oh, I see. It would be too complex given those relationships you mentioned."

The problem both my friend and I are dealing with:

The one causing issues is HIS SISTER. She's one of those "I can't do anything wrong / I'm always right / It's never my fault" people. Whenever the parents (the owners of the restaurant) leave her in charge of using the computer and adding data to the current app, bad stuff happens. Whenever my friend comes with a new order, she goes to the kitchen and tells the chef what to cook, but she forgets to add the order into the system! By the end of day, they have counted their winnings in $ X, but the system says is $ Z. Well, duh! SOMEONE didn't go their job! When she's confronted by the parents and his brother, she claims she did type all the info and clicked the "Add Order" button, but the system didn't do it because of a VIRUS OR A HACKEEEEEEEEEER!!!!! (A computer that is never conected, by the way.)

She doesn't even know what a virus means; it's just her little crutch to justify she didn't do anything right. If her brother points out the system has never failed to add stuff when he has used it, she pouts and mutters "Maybe is a virus that comes and goes..." If he points out the computer is safe from hackers due to not being connected, she stays quiet, but quickly goes back to her old ways.

This problem caused by her has been present in every app they have used. How can we (my friend and me) explain to her is not the program, but HER? If / when I finish my app, she's gonna blame it too; no matter how good I made it, it would still have "lots and lots of viruses that don't let her save stuff."

We feel that even if we teach her the difference between a virus and a software with a bug, she's gonna scream it's "has lots and lots of bugs, then."

If someone asks her: "Hey, I heard soulblazer made a program for you. You know, I need someone to make a program for me. How good was yours?" and she starts her rant about "viruses" and "being unable to save stuff", I'm gonna sound like a thief. I don't want that, neither does my friend.

Other than convincing the parents to ban her from using the restaurant computer and changing her tasks to cooking the food, or asking for orders (we don't want her to be fired over this), we don't know what to do.

Answer 1

In my contracts I have a field that tells: "company representative", that is the person who is officially authorized to communicate with me about the project and with whom I talk when I need info and stuff.

If I were you:

1. I would meet your friend and tell him that you need a company representative, or contact person, who is "computer aware". You will only and exclusively talk with that person, everyone inside the company will be able to communicate with you only through this person.

2. I would talk to all of them and say clearly that your app is tested and working perfectly, but in the remote case that there will be a bug or a problem the company representative (#1) will privately report it to you and you will make tests and solve the bugs.

3. If there are viruses or hackers is a company problem, let them call a technician and fix the computer/network/etc.

4. Anything that will be said publicly about malfunctions and possible problems about your work will be evaluated for possible sue for defamation and indemnify request. Anyone who wrote in social network or said publicly anything like that must delete the posts and fix the problem immediately.

5. Add Google Analytics, or other stat service, to your app so that it will log the real accesses to it, so noone can tell that they added an order at time XX:XX when the logs shows no access at that time.

6. Do your best to make your app stupid-proof to avoid any possible predictable problem.

Answer 2

With all due respect, doing this for free is not a wise idea. Western culture doesn't value stuff when it's free.

Were this family to actually purchase a restaurant point-of-sale system:

1. They'd be required to decide on features that they need or don't
2. They'd be on the hook to make a purchasing decision, and commit to their decision (i.e. not decide, the day after, that it wasn't appropriate for them)
3. They'd be on the hook to arrange an opportunity to review the software before using it in the business.
4. Most importantly: they'd be on the hook to PAY for additional training, consulting, or modifications

You have someone unreasonable on your hands. If this were me, I'd refer these folks to either an open-source product (do NOT offer to install or otherwise support it!) or a company that provides software and support. Take yourself out of the middle because over time this will only prove to be a drain to your morale, and it will wreck your friendship.