Plus One Non-Married Co-worker Etiquette

For our wedding we are giving plus ones to anyone who is married or living with their partner as well as people who have been in long term relationships. I have a plus one dilemma for one of my prospective guests. My coworker, who I have a relationship with outside of work (as in group activities with coworkers off the clock). He’s also one of my good work friends generally. Talk about life, wedding planning, etc. He has a girlfriend for a few months now, maybe 6? Not even sure. Never met her and probably never will. We don’t really intersect our social circles much. Is it ok that I don’t extend a plus one?