Dont assume its her man, it could be her relative or a friendzoned buddy.
The majority of time her being with a guy is not even a threat from my experience, and if it is her boyfriend she will tell you in the first minute. You will usually get a verbal warning from the guy if she is his girl. Dont assume a guy with a girl ever, i have alot of female friends that hangout with me and people assume we are together when we are not attracted to each other. You should not be intimated, since you are getting a conclusion from an assumption.
Even if she has a boyfriend, it doesnt matter at all..(unless she has a ring/fiance) but thats for a different post.