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But honestly, I'm not too fond of co-op play myself, and I haven't tried connecting to any PC player's game since v1.8.5 came out. So .. "supposed to work" and actually working are (sadly) two different things.
You might also want to make sure you're both correctly logged into the SHiFT system.
https://support.gearboxsoftware.com/hc/en-us/categories/200330390-SHiFT
If all else fails, you can try downgrading to a previous version (note that you both need to do this, you must be using the same version of the game to do co-op): https://steamcommunity.com/sharedfiles/filedetails/?id=2014284368
Depending on how old your girlfriend's Mac is, she might be able to swap over to Windows using Bootcamp (older Intel Macs can just do that, the process of setting it up takes a while, but is fairly painless). However, if she's using one of the newer M1 or M2 Macs, then she can't run Bootcamp. She could still run Parellels but ... I've personally never managed to get that running well enough to run BL2 and do co-op play (again, just not much of a priority for me)).
https://apps.apple.com/us/app/parallels-desktop/id1085114709?mt=12
(And honestly, it's really kind of amazing that BL2 for the Mac got the v1.8.5 upgrade at all, given that 2K/Gearbox didn't want to pay Aspyr to do the work. BL:TPS never did get the upgrade and no longer runs on post-Catalina systems (much to my personal annoyance)).
iirc BL2 on Windows is 2.0.0 and on Mac its 1.8.5, you can check the versions by going into the Matchmaking Server Browser screen in-game.
https://support.aspyr.com/hc/en-us/articles/203396134-Borderlands-2-Mac-FAQ
"Q: Can Mac and Windows play multiplayer together?
A: Steam users can play online multiplayer together when Mac and Windows versions are in-sync. Currently Mac and Windows are not in-sync and cross-platform multiplayer is disabled."