What to say when she tells you she has a boyfriend:
Her (in a playful way): "I have a boyfriend" -
You: "I can totally relate...I actually have two kids."
She will more than likely call you out on it. But more importantly, she will be changing the subject back to you and probably realize that her being in a relationship is not that big of a deal.
Her (in a defensive way): "I have a boyfriend"
You: "Wow, I'm sorry, you must have thought that I was hitting on you. I was actually just being nice..."
This will make her feel guilty and she will more than likely apologize to you.
Her (in a bitchy way):"I have a boyfriend."
You: "Great. Can you do me a favor? Tell him to go fuck himself."
Use this last one only if you really have to.