What would you do??

I thought i was close to this one person I considered a really good friend who also is a really close cousin of DH. Well i thought i could trust this person with the way i feel about my MIL because i have had many issues with her my whole relationship. But we were on civil terms with each other. Well me and MIL ended up being in a disagreement with each other on something i feel very strongly about. It ended up into a full blown thing to where we do not speak to each other at all. To me it shouldn't be that big so i decided to apologize and in her response i end up find out that EVERYTHING that i have vented about to this "Friend" was told back to her! This is the SAME FRiEND that came to me letting me know what was said about me and allot of other stuff! I REALLY want to confront this person and call them out to being so too faced! But DH doesn't want me to start more drama in the family!! What should i do?? I really want this person to know how i feel soooo bad!!!