That's a Sys6. IIRC, the default coin mechs will have chutes/arms that are fairly enclosed in plastic. There will be a triangular shaped button on the top of the enclosure that will manually register credits. If you don't have the OEM mechs, which is entirely possible, you'll have to figure out how to manually trigger that arm in order to test it. That is, assuming he's been using coins, and if he doesn't know how to debug the problem it's likely he has been.