T. has nailed it, unsurprisingly. This is a fairly straightforward issue. As a general rule, most Central Nervous System (CNS) Depressants are contraindicated in terms of combinations. One of the primary issues, is that combining multiple CNS depressants often produces a powerful synergy in effect. The easiest way to explain this is that 1+1=3, not 2. This goes without saying, but the more drugs one takes, especially Benzodiazepines and Ethanol, the less able said person is going to be to control themselves, most importantly, they will be more liable to consume more drugs either by carelessness or through legitimate amnesia.
I'm not a fear-monger or at least I try not to be, but it wouldn't come as a surprise to me at all if someone were to tell me that they had lethally overdosed from this combination. It's totally possible and the combination is not to be fucked with. You mentioned that your Father becomes visibly intoxicated following consumption, but how bad are we talking about? Some details would be helpful. Is he falling asleed in awkward positions, in mid-conversation or even when standing up? What do his eyes look like?