Okay, pwhodges has a point about Jane
So I'm revising my story to: Jane is an old friend of Henry's, who meets Veronica through Henry, and either
1. She gets into the business around the same time as Veronica (which leads to the friendship/rivalry dynamic with Veronica that we just saw). or
2. She is already in the business when she meet Henry's new girlfriend, Veronica (and the fact that Veronica is in the same line of work as her helps build their friendship, but with just a bit of rivalry)
I don't think it odd that Veronica has been invited. If they weren't on good terms it'd be odd, but it's been mentioned many times that Henry and Veronica are friends. My mum is on good terms with both my dad and his new partner. If my dad eventually decides to marry his partner, I'd be more surprised if my mum didn't get an invite than if she did. (We've already had Christmas one year at mum's house with dad, his partner, my brother, my brother's girlfriend, my other brother, and I). Maybe my family (and Marten's) is the exception not the rule though.