It's hard to say without knowing the frequency & tone of the comments.
Why would she have a playdate right before she knew they had to be at your birthday party? Who does that?! And then makes excuses as to why they were late. That would annoy me, too.
If she is being catty with the comments, then she's really an insecure person trying to make herself feel better and prove to you that they're "in demand". In my experience, opening up to someone like that will only make you seem like you're insecure or jealous, because she wants to be "better" than you, the one in control. No matter how you word it, she will still think you have sour grapes.
Are you really THAT close? Or are you more of acquaintances that know each other because their kids like each other? Because it kind of sounds like they're moving on & you aren't as high as you used to be on their priority list. I don't know that I'd put much more effort into it, personally. Friendships shift, change, or end, and that's okay.