Why don't you ask her to just suck the other guy off while you lick her pussy or fuck her? If she is into oral sex, she might like to try that out while she is fucking the man she loves.
Who knows, she may be so wet and into it, she may agree to let the other man fuck her too. Worst case sinario, she gets a mouthfull of cock and her pussy filled by you. It's a win win...
