Statistically, it's likely that your student will have a wonderful, memorable, formative experience. There is only a small chance that it will be otherwise. AFS is equipped to handle three scenarios well. Smooth sailing is, of course, no problem. Minor issues, stemming from cultural differences and the normal issues associated with teens, can almost always be resolved by the host family and student talking the matter through. The third scenario, in which the student is severely injured, becomes seriously ill, or commits an egregious breach of the rules (driving a car, dating) ends with the student being sent home. These cover the majority of likely situations.
In this forum are reports of rare but real scenarios which AFS, because of it's fragmented structure, or because of differences between the cultures and laws of different nations, does not handle to the satisfaction of parents. Examples posted include students being robbed, raped, or suffering severe mental stress. I add placement with an abusive host family to that list. In these situations AFS tries to fit the problem into one of the three scenarios they're comfortable with. They advise the student to talk to the family, they warn the student to behave, they threaten to terminate the exchange. In all cases they assume the student is at fault.
These are inadequate and harmful responses to these sorts of problems. As these students and parents have found, they have little or no recourse. AFS does little, probably because there is little they can do. It is not a monolith with a chain of command, at least not across international lines. It is a constellation of similar, affiliated volunteer organizations.
This is not to say that nothing can be done, just that nothing is usually done because it's difficult.
This doesn't mean AFS is off the hook though. These are our children we're trusting them with. Parents rightfully expect more than deflection and stonewalling.