29" freewheel ruined (slipping)
I'm betting that you need to install a brand new chain and a brand new freewheel / cassette, at the same time, and out of an abundance of caution, make sure they're the same brand. I doubt your friend actually ruined anything, the more likely scenario is that the chain is old and stretched, causing it to slip ( or it's the wrong length as Daniel mentioned ). This isn't all that unusual.