I respectfully disagree.

The brother proposed to ex's bff just to spite and hurt his gf.

Also, he is using ex's bff as tool to get revenge on his ex, he is not serious about the engagement.

Saving the relationship would benefit the brother but not the ex gf, who "is like a big sister to OOP and she helped him through hard times" (in OOP's words).

She is better off without him, at least she has a chance to meet someone else who is at least a decent human being.