What were the mistakes that were made? Why are they/he afraid that sex will ruin their friendship? Were you and he sexual with each other, and, if not, why does it matter to you so much what they do or don't do? Were you hoping that this would develop into a full-blown co-primary relationship, in which case you've sort of lost the potential to have a brother-husband, so to speak?
It's hard to know what to say without more context.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.