I think your last few sentences said how you're truly feeling...
"Is there a cautionary way of saying 'get over each other. there's plenty of fish out there. why do you have to choose my friend/exbf?"
I understand it's hard to see your friend and ex-boyfriend together, but if you're understanding of their relationship, they'll respect you more.
The truth is, depressed or not, they like each other, and unfortunately there's nothing you can say or do that will change the way they feel about each other..if anything their feelings will change about you.
My advice..let them be, and as hard as it is..give them your blessing!
My best,
Brandy
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