It's pretty normal for anyone, male or female, to do that because a partner isn't just a romantic interest but also usually your best friend as well so of course you are going to want to spend a lot of time with them. However that doesn't mean it doesn't suck for the friends left waiting, and she shouldn't be ditching you entirely. Just talk to her, let her know you're totally ok with her spending a lot of time with him but that sometimes you feel a bit abandoned, as if you're just a filler for when he's not there yknow?