What do you think about Alice? What do you think about Frank and Marry?
There's no objective right or wrong here (at least initially), there is just social convention, which we might call the 'default' right and wrong, and then the personal right and wrong as defined by each individual, and the conventions of each relationship Alice/Frank, Alice/Marry, Frank/Marry etc, as agreed (whether implicitly or not) by the two participants in each individual relationship.
Social convention generally is that Alice is wrong, but Marry's actions are fine. Individual right and wrong vary by individual, but it is often the case that people are selfish and want everything their own way... and they then use this feeling to attempt to redefine right and wrong. Stupid and insincere, but happens all the time... and most of us are susceptible to this to at least some extent.
Simply in this situation I would suspect that Alice just doesn't like Marry being with Frank, and is attempting to redefine what right and wrong mean in order to justify her position... in which case Alice is objectively wrong, because she is changing the terms to suit her preference. Everything else is subjective.
Very nice said. Alice's behaviour is selfish and she is trying
others to accept her subjective point of view (in whatever cost). Why does humans behave like this? I am sure that this kind of behaviour is quite common as you said, but I am consider it as a little bit... pathological. Alice definitelly would not like if others will behave the same to her, so in the end, this behaviour cannot bring her anything positive. What do you think?