My friend's boyfriend invited me for lunch?

So I met this guy through his girlfriend which is my friend, we are not very close friends but yeah we get along together. I realized that this is the type of guy who makes friends very easily and is very easy going. This guy sent me a friend request on social media and he replied to one of my stories. I politely replied and we started having a casual conversation and he invited me to have lunch with him which I haven't answered yet because I honestly don't know how to act at this point. I know he doesn't have any other intentions other than to have lunch because he's in a relationship with my friend but the fact that he's never mentioned if she's coming with us or not is bothering me and I feel like if I go have lunch with him I'm betraying her trust. How would you deal with this situation? At this point he already asked me twice I ignored his first invite but he keeps insisting.
My friend's boyfriend invited me for lunch?
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