Sketchy or no?

So ladies, do you think it's odd for your man to send his ex wives female friends, friend requests on social media? This friend was not a mutual friend of theirs, just his ex's. I find it very odd and not sure why he'd even want to be friends with her. Of course he says I'm crazy.. am I over reacting or is it sketchy?

More info: him and his ex wife were swingers. 99% sure they hooked up with this girl during that time