You've implied by your over-reaction that any contempt for 'x' on your part arises out of jealous insecurity (which, though understandable, is nonetheless contemptible in itself), rather than from an aversion to his character and mores (which might in the circumstances be somewhat reasonable, assuming that he was originally supposed to be your friend). Of course, if you actually trusted her, the presence (and previous overtures) of said individual would be irrelevant; he cannot 'get her' to cheat on you, because to 'cheat' necessitates a voluntary undertaking on her part. So you've essentially demonstrated that you trust her only inasmuch as you can reserve the right to outright prevent her from seeing men that might pose a (perfectly legitimate and acceptable) challenge to your consensual monopoly over her affections; which, to her, will feel about as offensive as it does degrading and hurtful.
Call her back in a few hours, and apologise. Admit that you over-reacted; that she's right (because she is); and that inviting him along purely in order to make up the numbers would be perfectly acceptable, with the obvious caveat that he can't be trusted to observe the boundaries of friendship between the two of them (hence your initial concern, naturally).