Just a kid thing?

So my youngest kid is really a go with the flow kind of guy. But if you piss him off (and you aren't family) he will hold a grudge against you. He does not put up with being messed with. There is this boy in his class that he somewhat befriended. Things were going well it seemed. The boy has to take medication for "behaviors". He would come over and play xbox and spend the night. Other mothers warned me that he was weird and somewhat "neglected". Their words, not mine. Anyway, this kid would come over, I'd give him rides home from practice, from school, etc. But if he doesn't take his meds he gets all crazy and takes it out on Zach. Treats him like crap. Zach got tired of it but knew I felt sorry for him so kind of dealt with it (I didn't know the extent of the craziness). The final straw for Zach was a few weeks ago when the kid called him names and threw basketballs at his privates. That was MY final straw, too. I told Z that he just needed to tell him no if he asked to come over or if he wanted a ride or whatever. So today after the basketball game he calls my cell phone twice. And I ignored it. And I didn't tell Zach. And then I felt guilty. So...is it just a stupid crazy kid thing and should I continue to feel sorry for him and "let" them be friends again? I just want to protect Zach and I don't want to be taken advantage of, but I do feel bad for this kid that seems to just have a crappy home life. Blahhhhhhh