Вопрос

I've never been in that situation before. But most likely, I would just be a little upset, and still try to stay Друзья with her.

I'd stay Друзья with the guy/or girl . I'd still hang out with him/her , & maybe hang out with their girlfriend/boyfriend. Anthing could happen .

This boy I was TRULY in Любовь with did this many times. I told him my real feeling,then he сказал(-а) his.{he was also in love} We started off being very good friends. NOT DATING! All the girls he dated HATED this so much. Same with all the boys I dated. We later started dating and still are.
My point is...tell him или her how Ты feel. Then become good Друзья and see how it would feel 2 дата them.

Cry... burst into tears. Write something that sounds Эмо to express my feelings.

thats always hard, but, out of respect for them, I'd leave thier relationship be and try to befriend them,it's better than nothing