How to deal with an over-talkative alcoholic who has no manners?

This is the situation: My girlfriend’s parents run a restaurant in Greece , my girlfriend works there too.
I am visiting her for some weeks and when her shift starts I accompany her and do some little jobs.
Almost every day this guy passes buy to drink one, sometimes two beers (we are the umthiest restaurant/bar he visits every day).
He’s a fellow Dutchmen, but he lives there.
He likes to chat to me, but his talking is so slurred, slow and boring/uninteresting that it drains my energy (energy I rather spent on my girlfriend).
It can take literally minutes to simply tell me about how much a car cost that he owned thirty years ago…
He is on obvious alcoholic and seems to have lost all understanding of etiquette and/or social behaviour.
When I start a conversation with my girl he feels no shame and interrupts us.
The simple thing would be to bluntly tell him that I don’t like his behaviour, i know, but he is after all a customer, a customer of my parents in law’s restaurant…
Any Jellies around with wise advice?!
It would be much appreciated.