Firstly, I'm assuming your friend is a qualified agronomist and knows how to do the job, with that said; part of the role of an agronomist is typically field-based work, so they may need to be prepared to travel or make alternative arrangements for soil sampling, field scouting etc, that part of the job aside there's no reason it wouldn't be possible to work as a "remote freelance agronomist", providing they are able to find clients.
As with any freelance role they'll need to consider the way they position the services of an agronomist. The Alberta government list a series of things for farmers to consider which might make a reasonable starting point for working out how to pitch the role to clients.
It's also worth noting that many farmers may not be aware that they have the option to hire an Agronomist (whether a freelancer or part of a company), so your friend may want to consider cold-calls, advertising, or writing on related topics, in order to generate leads. They should also try and get testimonials from any past-clients in order to establish that they have experience in this field and aren't straight out of University.
Finally, in terms of resources I'd suggest reading around outbound marketing techniques (particularly traditional approaches; cold-calls, direct mail etc…), looking into local trade shows related to farming/agronomy that they could attend, and potentially joining a trade organisation (if they aren't already a member).