Eh I see both ways in this argument, but I’m more inclined to think it’s a dick move by her. On one hand, her not telling him the reason for the breakup can be reasoned by the fact that she’s in the closet and it’s not comfortable for her to disclose it.
But, take one step back and observe the situation, you’ll see that this makes no damned sense. So she’s been with this guy for 10 years, and they’ve been intimate to the point where he was planning to propose, but she still didn’t consider him enough of a trustworthy person to say that to? But wait, she obviously does, evident by the fact that she’s telling her very gay brother to go confess to him, which means she believes that he’s not that type of person.
So why? Why didn’t she tell him? What did keeping him in the dark even do except for leaving him hurt and confused?
Though, it’s most likely that the author just missed this perspective and didn’t mean to paint her as a bad person, but imo her actions are pretty hard to defend when you think about it.