SO with female friend

Sarah
Just wanted to gather other opinions. My SO has an old uni friend who he has stayed in touch with along with a few others but this girl, very clearly to me and his mates, rather fancies him. I've met her a few times and she's always been nice to me etc. 
But she tends to overstep the mark a bit. A few days ago she invited my SO to a wedding as her plus 1. That just strikes me as inappropriate. I have no issues with him seeing her as I trust him but a plus 1 is surely for romantic interests not guys with girlfriends. He's told her he won't be going and his mates have said to him that it's odd of her to ask.
Just wanted outside opinions. Is she after him? Am I over reacting?