I think Op has the right to this, since he/she's taking care of the animal better than what it might be getting elsewhere... and it also depends on whether the animal is willing to follow him, if not, I will just advise he begins to reduce the amount of food or care he gives it until the day he leaves. That will give it the opportunity to adjust gradually and able to cope when it nolonger sees its care giver.
I once in a while take care of a stray mother cat and her child but I let them be for now because they are ferocious hunters and can be quite independent.