It’s hard to say. Has she come on to other people before when she’s drunk or was it just you? Is she satisfied and happy in her current relationship or is she only staying in it because she would feel guilty, (or he would make her feel guilty) if she tried to leave? In my experience, if someone who is in a relationship tries to kiss someone who isn’t their partner, then there are issues that need to be addressed, and at least one member of that relationship is going to be unhappy long term if those issues are not discussed.
I don’t think you’re a bad person for suggesting she not tell her boyfriend, however well-intentioned though, I don’t believe it was in her best interest. If her relationship is going to work and be happy long term it must have honest and open communication. Despite what some people believe, preserving a romantic relationship (even a longstanding one) at any cost, isn’t always the right move. In my opinion, if you have to lie to someone to preserve a personal relationship, then it’s time for that relationship to come to an end. Either the other person can accept and care about the real you or they can’t, and if they can’t accept you for who you really are then why are you wasting your time?
Not all relationships will last until our lives come to an end, more often than not those unsuitable relationships slowly fade away as each person slowly changes, and making a relationship work successfully through these slow changes on both sides requires a lot of honest communication and understanding. However, there are also cases where the changes come rapidly and the relationships can die violently. One example of that can be seen when a JW comes to an epiphany about their beliefs and needs to leave the religion, a large number of their relationships can evaporate over a short period of time.
Romantic relationships can be especially bad about outliving their suitability, especially after they get to a certain point. This is because in a romantic relationship people can get tied together by so much more than the relationship itself. They might live together, have joint bank accounts, share a cell phone plan, buy a house together, have kids, etc. The more ties like that there are, the harder it is to get out of a relationship that is no longer suitable, and it’s even possible to end up with all of those things in a relationship that was never a good fit in the first place.
Romantic relationships are very hard to get right, especially at a young age. My theory is that this is related to the tendency to be dishonest during the early stages of courtship. Now I don’t mean overt lies when I say use the word “dishonest” here, rather I’m referring more to the tendency people have to repress who they really are in an attempt to present someone they believe would be more appealing to their partner. Sometimes this sort of repression can go on for years, all the while they stay in the relationship and build those extra-relational ties I mentioned, not because they are happy and allowed to fully express them self, but rather because they are simply afraid of being alone.
Sorry, I know that’s probably longer than what you’re looking for, I’ve just thought about this topic a lot.