Try and find out if any of the fish have been sick or if some in the tank have died. I'd be very nervous about using media from any tank which was as poorly cared for as that one...but it's also a sin to throw away healthy
(< I like that
). If you have any doubts, don't use it and start fresh. You're also getting some very conflicting info from that person...so I'd personally question everything they say.
As for cleaning...research some different methods online. Some use ammonia to clean (which you just so happen to have
), or you can also use a mild bleach solution. I believe the ratio is 20 parts water to 1 part bleach. Obviously be sure to rinse it extremely well and use a strong dose of dechlorinator when you fill it. Just don't let detergent anywhere near the tank.