EDIT: Move on, this was a stupid question. Don't let a client's relationship to another client's get in your way: be clear, and direct, as usual especially regarding getting paid. And don't ask stupid questions if you want rep :(
So, I was commissioned to help fix a residential network - someone paying for cable, phone, and internet had issues with using the phone and internet at the same time. (No, it's not dial-up, its cable! haha)
Anyway, the ISP in question really dropped the ball. The client was supposed to have a tech show up to replace the modem - twice the tech was no call, no show. Client reports this, and is told he can go get the modem himself. He asks me to take out the modem, go get the new modem, set it up ... he would rather pay me than pay the ISP for the work they didn't do. When swapping the equipment, the ISP literally had nothing to say about the no shows. They didn't ask any information, just took the modem, and sent me off. I asked about hooking it up (I already knew how, wanted to test their knowledge) and they handed me a pamphlet and (incorrectly) said "plug it in and wait 15 minutes and you're done". Pamphlet, and experience, says otherwise.
Well, the modem wasn't the issue. Old one worked same as new one. The issue is that for whatever reason, these modems only allow one phone line in. Any more, and the modem resets periodically, and resets when line 2 rings. So with line 1, he can manage, but this leaves a sour taste in both of our mouths. This is all the ISP would have done, and charged him for it, but they didn't show, or care.
So I was able to solve the issue, on his dime, and (somewhat) understandably, he has asked me if I could seek payment from the ISP. Not demanded it, mind you, just asked if there was any way... so I politely tell him, unfortunately you still owe me for services rendered, and would have to seek compensation for the bill from his ISP.
I could have lied at this point, said just call them they will repay you, I'll take your credit card and run it please. But I want to be a trusted consultant, so I tell him: "I'm not sure how to go about this off the top of my head. I'm sure they won't listen to me, some freelancer. But I'm not sure what to tell you to say, beyond the obvious fact that you and I just did their job for them, on your dime."
I did what I thought I should do: gave him a grace period so we could both try and get payment from the real culprit. Left him an invoice due in a week, and told him that I would ask a trusted friend (stackoverflow is that friend :D) about the issue. I would LOVE the chance to stick someone at this ISP with the bill. They have caused me and my clients nothing but grief. Fixing phone issues is NOT what I had in mind for winning bread! Plus, this guy is well connected to a client of mine, so I'd like to be as much of a champion as I can, and not just another bill because the ISP stinks.
I think there is a snowballs chance in hell this guy could get refunded for the money. I think there is even less chance there is anything I can say or do about it directly, or else I would go back to the shop that I got the modem from with the invoice addressed to the ISP. But whatever advice I can give my client that will get him refunded would be fine. I guess I'm looking for a way to provide that extra level of service, the warm fuzzy feeling - plus, fixing something for a client for free almost certainly guarantees a stellar recommendation!