1988 ford f250 clutch not working

I am trying to help my father in-law with this problem.He got the truck stuck in the snow and in his words"used her a little"to get it out.After this the truck started reving more and more,but the truck went slower and slower.It has the internal slave cylinder on the hydraulic clutch and I checked to see that was working.Has he burnt the clutch disc out it?

Can't be positive, but that's what it sounds like to me.