With engine not running, dashboard ammeter lead sparks when touched to alternator "+" terminal. How come?

The alternator is for a marine diesel engine in my Dad's boat. It had starting problems and he had work done on it to much cost but no success. I strongly suspect they are ripping him off.

Anyway, the deal is that one guy disconnected the dashboard ammeter because he thought the long run between the engine and the dashboard would drop the charging voltage too much. Well maybe, but all he did (apparently) was disconnect it at the alternator + output, not bypass it with a new lead direct from battery to alternator.

Since then, they replaced the alternator with a new one. My Dad asked for the ammeter to be put back but when the guy tried to - with the engine off - the lead sparked against the alternator + terminal. Since then my Dad has tried this himself and confirmed that it sparks even with the engine off.

This makes no sense to because a) how can the battery charge if there is no connection to the alternator? b) How can the lead spark when the engine is off? The only source of current is the battery but surely the rectifier diodes in the alternator block any current into the + terminal? The voltage regulator is internal to the alternator.

Can anyone explain this other than they put in a dud alternator?