IF it is only the roof then maybe a good deal, IF it is free.
Remember deals involving money between friends(or family) often wind up going south along with the friendship.
How are the walls? Mushy? Solid? Can they hold a screw? What about the floor? Soft spots? Mildew smell? Carpet or linoleum? Plumbing? Will it hold 60 psi? Axles? Spring hangers? Bearings? Brakes? etc. etc.
If you like spending your time fixing stuff then it might be worth it to you. Maybe 30yrs ago I would have tackled that project, not today. I use to rebuild my car transmissions and motors, now I sell them as junk if they need a rebuild.