Cheating, or "just friends"?

Sooo this is a weird type of question but I need to know if I'm out of line with my thinking. My "bf" (such a complicated situation it's not even worth explaining) and I have been together for 2.5 years. We haven't seen each there much since I went back to school. He is 28 and lives at home with SSI, he works with this girl who is his boss - he's only worked at this job about a year. His birthday was recently and on Facebook (I know it's so dumb 😞) his boss (girl) commented "happy birthday, love you :)" I got offended and defensive because he has cheated on me with his ex at the beginning of our relationship. When I confronted him about the posting and why she  might post something like that he was very abrasive  - telling me I always want to fight about something and that he "didn't say it back and they're just good friends so why does it matter". I felt as though he was bullshitting me. It's unusual for an employer to say that (I thought) am I crazy??!!?