I am not in the "industry" but I do own a horse and have been active for the last 17 years riding.
A. Why do people always get rid of their horsies when college comes along? That makes me sad. I have one more semester and I managed to find plenty of time for my horsey in college. Honestly, it's like getting rid of a member of your family.

Makes me sniffle.
B. It's ALWAYS hard to sell expensive show horses. 17500, is quite a pretty penny for a western pleasure horse (a Bay at that... I know that sounds stupid, but i've been told a lot of people will pay for some flash in the western show ring, a nice paint perhaps... so on so forth depeneding on the trend) Not that he isn't worth it, but it's a very small market compared to the pleasure horse market, or for that matter the english showing circuit. And he's 11, so he has a few less prime years then some other horses in his range (I know 11 is still plenty young, but compared to someone selling an 7 or 8 year old, a few years is a big thing)
C. I see he's located in Indiana. Perhaps she hasn't broadened her posting site enough. Indiana isn't exactly a rich state. More western pleasure, but still, selling a show horse could be harder. You go south, more people are breeding their own show horses, and if it's a gelding, that's less of a use for someone too as there is no breeding possibilities. I'd tell her to broaden (if she hasn't already) her postings to perhaps... richer areas. Northern Illinois for one, (out and around the chicago area, surprising amounts of people into showing up there that have the money to spend on a 17k bombproof "kids" show horse).
D. Along with point C - perhaps try mailing out some of those flyers (20 or 30 or so) to a variety of barns in said areas. Just do some googling with "western show barns" or something along those lines and write out some envelopes and spend a few bucks on stamps and send those off to the owners with a note asking to post the flyer in their barn. It may be out of state or something, but atleast you'd be reaching the right crowd and could get some good hits, and a lot of barns have email lists, if she sends an email, a lot of owners would be MORE then happy to ship the email ad off to all the boarders there. A good way to get it to circulate to the right crowd.
E. after all is said and done, it just isn't horse buying season right now. It's winter. Most people think about buying horses and training with a new horse in the spring..... maybe february/march at the earliest. Show horses are a big investment and a very thought out thing typically, not usually christmas presents. Pleasure horses, different thing, but I think she's targetting the wrong crowd. I know she's probably thought this through and all, but perhaps she should rethink if she really won't have time in school to work with her horse at all, it really is a great stress release, and unless she needs the money from selling her horse, or she cycled through a few horses in the last few years, it might not be a bad idea to hang on to him. It's hard to sell a horse
F. Goodluck! Hope something in this helped!