Hi Sara :o)
It could just be that he is wanting to increase his friends list so it looks like he has lots of them, he may not even realise or know the connection between you and his girlfriend. He also could have sent another 50 requests that day from her friend list for all you know.
I totally understand where you are coming from about wanting to be the one who is being fair, I'm the same, we need to toughen up together lol
Just don't accept it and move on, it's your choice no-one elses and you shouldn't have to explain yourself to anyone :o)