YANBU obviously. As has been said, do NOT answer the door to them OR arrange to do anything with them again. Rude bastards. You could have arranged to do something else. 
If your partner's mate does not have form for doing this, then it's definitely his girlfriend's doing. Probably did it on purpose.
This puts me in mind of a couple we used to know (let's call them Ed and Paula,) who we arranged to meet at the pub on 7.45pm on New Year's Eve several years back. (2014.) They had form for being late, but this time only SHE turned up - though 45 minutes late. 
It was a bit awkward because she was a bit hard to talk to, for various reasons, but if he was there, it was a little bit easier. We asked where he was and she said he is a bit ill and will be in the pub later.
Half past eleven the fucker turned up. 'Oh I fell asleep on the couch' he said.
Months later (around May,) I stumbled upon a facebook profile of someone he works with, and has a nose at his photos, and lo and behold, there was Ed, large as life, at someone else's party on New Year's Eve 2014.
When I investigated further, it turned out he was there from 8pm to 11.15pm with a bunch of mates from work and a few neighbours. Apparently, he had had a 'better offer' than spending New Year's Eve with us, and she had refused to go with him, as she didn't like the other people, and they had arranged to meet us (first.)
So he went on his own and left her to come meet us at the pub on his own, and then decided to rock up to where we were at 30 minutes before midnight.
Needless to say, we have never been out with them again. Not only because he was such a rude fucker, but they were always late anyway, and were pretty hard work. (Being with her on her own was VERY hard work though.) In addition, he had not turned up several times before, and also she knew he had fucked off out with others, and she had lied. So after that, we washed our hands of them, and thought 'fuck 'em, we deserve better.'
We don't miss them.