Strickie

Alright long story short, I left my bike at home after last summer and left it to a friend to ride and keep maintained. When I was riding it I only experienced the "skipping" once, I tuned my derailleur then everything was fine. My friend called me today as my bike will be shipped out here next week to give me an update and said over the last month a few of the gears have been skipping really badly, he got it tuned up and put a used chain on it because the last chain broke. Is it possible that it is the chain being the wrong size or are the cogs worn out or anything else ??? I got him to message me some photo's so that you guys could take a look. If it is too worn out which it looks like is a very real possibility what are my options for replacement. I have c record hubs and the current cassette is 12-21 Im not sure what family it is in but it is campy and came with the hubs when I purchased them. It's and 8 speed so can I use any old campy 8 speed or are there things I have to watch out for, I just really want to have a cassette for when the bike arrives just in case and want to know what to get, but don't want to blow 100+ bucks on a cassette that won't work with the bike. ????