How long is this supposed to last for? Would this be a permanent relocation?
You are being very smart about not jumping on this and thinking it through. I have done this before for some people in the past, and it's always been a great experience. Everything to consider, has already been mentioned above I think, but I'll go through my own thoughts and experience's I've had about this and will post some of the same points again.
Doing this kind of trade with someone is great, because it's giving you a fantastic opportunity to get some really valuable experience, along with the free lessons and horse to ride. It's hard to get anything for free or in exchange of work or favor these days, so it's fantastic you have that opportunity. Having said that, you need to make sure you have insurance to cover emergencies or anything negative that might or could happen in the future. As you know things can go downhill VERY quickly. Should a horse escape or break a fence and head on a highway, should a neighbor climb into your field and get hurt etc etc...the list goes on and on.
You have to make sure that your friend, is a very good friend. You have to make sure you have a very good relationship with this person, because you are going to be working together closely. Tho it never happened to me, you have to make sure that you make your exchange of board and free lessons, horse etc very clear. Even if you start out strong with your friendship with someone, when a deal like this takes place, I've seen good friendships get wrecked over bad communication, or someone trying to use someone else or just bad behavior or lack of respect.
Contracts!!!!!!!! I don't care if it's the Queen of England boarding her horses at your place, you have to have contracts in place that clearly explain what both parties are responsible for and by what date. You have to have something in place should she stop paying for board, or quit feeding the horses, or causes you difficulties with anything.
If it's someone you really trust and can do business with, than this will be one of the best experiences you will ever have. I've gotten to ride some pretty incredible horses for free, or for next to nothing because I did favors to people, you just have to be very smart and very careful.