This being a crap place to ask such a question aside, there are a few more things I'd need to know before even attempting to answer that question...
What level of realism are you going for? The way you've phrased the question, you're not asking what we think it's realistic a girl would do, but what we'd like her to do. Did you mean it that way? Are you planning to portray her as a one-dimensional bimbo who wants nothing more than to be a fucktoy? In that case "Get on her knees, pull out her tits and open her mouth for his cock" is a possible response. Would we like some level of realism, but still have her portrayed as sexually aggressive and confident? Maybe grab his cock through his jeans then, or take one of his fingers in her hand and suck it while keeping eye contact.
Secondly - Context! How old are they, where are they, what are they doing, are they drunk, a rough outline of their personality, social structure around them, etc, etc...
