Who gets to decide on leave though?
Has your day off already been approved and he is now looking to be off at the same time? Does one of you need to be in work?
My initial take, based on the info you've given, is that you have already sought and been approved a day off on Friday.
Your colleague has sought, and been approved, many more days in September already. While being off today, and tomorrow, he has asked for Friday (not pre-approved) off as well - for a very specific personal reason. He assumes that you will roll over.
You are already approved, and have made arrangements for the day (although it wouldn't matter if your plan was to sleep all day).
Morally, in terms of supporting his GF, he has a case for the day.
But realistically, he is already taking a lot of days and this was not already planned. You made arrangements around his plans already.
As I said, my assumptions based on what you wrote.
I personally know I would feel somewhat "bugger off you twat" about it (based on the way you describe him) and that I am entitled to some time off too around his plans. Leave must be fairly shared.
I MAY feel sorry for his GF, and I MAY consider changing my appointments IF that was feasible. But I would also talk to the person approving leave and let them know that you are being unfairly pressured to come into work on Friday and that you are not happy about it.
I would leave it up to the manager involved though to make any final decisions I think.