If you had previously made plans for that evening, and she's now made plans for that same day in the day time, it's up to her to manage to meet both obligations.
I don't think it's so much a matter of her guaranteeing in advance that she's going to make your date, but instead one of you trusting her to make it. The more petulant you become, the less attractive a date you are (sorry, that's not meant to be mean, but I know it sounds that way). Just ask her to let you know when she's going to be meeting you, and then trust her to meet you. If she blows you off for time with him, then the trust was misplaced to begin with, and you're right to be rethinking.
I'd personally like to think that if you say "well, it's up to you to make it, just please give me an eta sometime that afternoon, if you can", she'd be responsible to her promise to you and make it to your date.
I know it's tough, but you're putting a lot on the situation yourself, and you're actually aggravating things for yourself. If you're not comfortable hearing that she may be intimate with him, perhaps you need to renegotiate or re-examine your involvement in the situtation, as you've said.