The first cheap thing to do it to take a pencil and put it inside each cut of the nut to see if that helps keep it in tune nothing to lose there...or maybe the nut is to high or going back. And of couse whats goin on with your tuners.. slipping maybe.. make sure your bridge is set, saddles...
Well the first thing I read was a friend.. so you and him or her can talk it out. Its what I do with my friends I make sure they are happy and we are both happy with anything we do. Good luck Hope it gets done..