You must also keep in mind, if the relationship is strong, happy and "healthy" one wont stray....A partner can not be stolen away, they have to be a willing participant..
Trust is huge, if you cannot "trust" them why be "with" them?
It is good you have great communication...you spoke to your partner about your concerns, he acknolwedged them even felt bad. In reading this I don't think you have to give it a second thought...you yourself stated he feels she is playing a dangerous game...which makes me think he does not agree with her actions!
Good luck to you both!