All my friends I must say are friends with benefits - they are true friends to me, they let me know when I am wrong, they put up with me-faults and all, they laugh with me and they cry with me and have even loan me funds at times however this relationship is shared between all my friends and remains healthy as it is not one sided and no one is giving or doing because they are being forced or required to do so. I would love them anyways and they me because that is what makes us friends.
So I guess it is fair to ask "What kind of benefits and with whom?"
Your friend can be opening a lot of worms if the benefits include money, gifts etc. and with a guy. Guys are physical creatures (no male bashing intended :) and they do express their feelings in a physical sense and unless the person's name is Jesus I really do thing he is giving because he has feelings for the person and hopes somehow that it is returned or will be returned over time.
This is also a sticky situation for you because she is your friend and you do not want to see her get hurt or someone else for that matter. Tell her the truth and let her know you are saying so because you care for her as a friend, if she doesn't appreciate your advise don't hold on to it just let it go and be there for her should she need a shoulder.
She should not take things from this guy if she has no feelings for him.