My neighbour's cat loves prosciutto. I get those mixed packs of antipasti, but I get bored of the prosciutto, so the cat tends to get what I don't eat.
Is it likely to do him any harm?

if it's once in a while it's unlikely to kill him, but it's not brilliant for them i don;t think

if it's every day then it'd be best to stop

No, it's once every few weeks.

A bit salty for him.

well it's not the best thing in the world for him, if it is maybe just once a month it'll probably be fine for him, but feeding other peoples cats is dodgy ground really, for you because the cat will hang around, and for the owner if the cat is on a specific diet.

and as mentioned above, it does contain a fair bit of salt as well.

maybe discuss it with your neighbour if the opportunity arises