Mods really don't increase the resale value. Sad for people like us, but true. It's why I can restore my entire bike to its original state quite easily.
Way way way to high. A year and a half ago I bought a perfect condition 1500 mi 2009 675. Granted that was pretty much the deal of the century, but my point is that it's been a year and a half and that was the newest model (for all intents and purposes there's a pre 09 model and a 09+ model). For the bike you linked I wouldn't pay more than $5000.
The seller mentioned that he'll give the buyer the original parts, so if he ends up buying it, unless your buddy really likes the mods I'd swap the original parts back on and make a bit of extra cash back by selling the modded parts separately. Especially the exhaust if it's not his cup of tea.
Best possible weapon for buying a motorcycle, ESPECIALLY during the winter, is being able to walk away. If you offer him 4 or 5K and he says no, then just say he can call if he changes his mind, but remind him you'll be shopping for a bike in the meantime.
