ok
It does look like someone has pulled the mech apart before, there is a broken retaining pin on the arm return lever. Not essential to operation but makes you wonder!
There are a few interlocking pins and levers under that large cog that can become out of sinc not to mention the latch mechanism and the secondary cog - it will probably need to be removed and reset.
If the large cog does not move then you could try diss-engaging the worm drive - this is engaged where the silver lever meets the brass springy lever we were looking at before. You should really remove the cog to achieve this though - I personally really enjoy doing this but I would highly recomend the service manual (downloadable silver member) and a great deal of care as everything has to go back correctly. Great fun working out how the mech works and I have a lot of respect for the folks who design these!
Of course you could (and should) complain to the person you got it off!