Under stress people often show their true character. I'm not talking about how people behave in the aftermath of an accident when addled by adrenaline, but how they behave when external pressures occur and they have to decide what their priorities are.
She may be under family pressure to have the perfect wedding, but her response is to bow to that pressure and pass it on to her friends. She has a choice to make her family "happy" or her friends and fiance happy. She is making a choice.
She is showing you who she is when the chips are down.
I don't think it's really that clear cut. Some people show their true character under stress, other people behave in entirely uncharacteristic ways - and for most people, it really varies with the situation. If someone were a long-time friend of mine, and they started to behave in an out-of-character way (based on my experiences) when under a lot of stress, I would not just assume that they were now showing me who they "really" were and that everything before that was just an act. That would be pretty uncharitable of me. If this friend had a history of tantrums and controlling behavior, that would be one thing.
If a friend was under serious stress (family, illness, money) and started crying or screaming or obsessing over various things, I would cut them slack. At first.
After a while, the test is not what the external stressors are, but how the person deals with them. Do they get councelling? Go for a run? Talk and laugh and cry with friends? Obsessively work on a hobby? Cut down on the scale of their ambitions All that is cool.
It is when a person deals with stress by selfishly outsourcing it by trying to control other people's lives (and have got some honest and kind pushback from people they love and respect) that it shows the character.