It's up to the character/player whether they keep the promise -- that's why it's 'promise them something...' and not 'give them something...' You aren't robbed of the results of your roll if the Ghoul decides to renege -- you're just overestimating what that roll gets you.
Everything in Monsterhearts is conditional, as a rule. This is especially true of social obligations and alliances. Most of the moves don't resolve anything at all, and Turn Someone On is probably the best example of the trend. Strings are temporary advantages, a resource to be spent -- if you had gotten a String and then spent it to try and get the Ghoul to shut his girlfriend up, the choice would still end up in the hands of the Ghoul (to take the XP or not, etc.)
I think when you consider the interaction in question as a typical teenage exchange, it becomes super obvious that the Ghoul does not need to keep the promise. I mean, what happened there? He kind of saw your boobs and it made him say something to impress you, in the moment -- whatever came to mind! What he decides to do about that later on, once he is no longer hormonally overwhelmed, will tell us something about who he is, and what he actually thinks about the way his girlfriend is treating you.
And equally importantly, if he decides NOT to follow through, you still get something: you get fictional leverage to get on his case. You get the fact that, at least that one time, he told you he would help you out. In high school that's something.