I would do two things. I would get a contractor in there to give an estimate of what it would take to fix the damage, and then, I would get an estimate from an exterminator as to how much treatment would cost. An exterminator can give you a guarantee that the termites will not return for a certain period of time.
I would take these estimates and present them with the adjusted offer (if I really want the home). If the home is not a big deal though, I would leave it and buy one that does not require the extra work.