These Short Term Tenancy Agreements usually make the renters jointly and severally liable for paying the sums for the minimum contract period. It means that if one renter defaults the others have the onus of finding a way of making up the whole amount, either by bringing in someone new or making up the difference.
Although your son was in a relationship with the other party, if the contract was set up in this way it doesn;t change anything.
Strictly he was liable to pay the rent, yet he came to an arrangement with his former girlfriend. Unfortunately he didn't think about the deposit. Another way to think about this is that his girlfriend 'let him off' the remaining rent due apart from the number of weeks covered by the deposit (often 6). Not much he can do unless what was agreed as he left was written down, I'm afraid.