Re: Cousin A and Cousin B..
Help her to get over B, distract her with something maybe shopping clothes? Watching movies? One of my closest friends was left by her boyfriend a few weeks ago, they've been togehter for almost two years and she is not a Muslim, nor is she some of those sleazy cheap girls he was her first boyfriend and he didn't even explain to her why he left her he just said "My feelings have stopped".
Your cousin A seems like a truthful girl and she was brave enough to admit her feelings to him, I'm sure Allah SWT has something good for her in mind, she is not in need of a loser like B. However it might take a lot of time for her to realise it, just stay firmly by her side as you've done all along.
As for B he will surely be paid back in some way near the future. Insha'Allah.