Teen asks for advice after discovering mom's secret relationship. [Updated.]

In a post on Reddit a teen asked for help with how to broach a touchy subject with his mom. Here's his story.

'How do I (17F) tell my mom (36F) that I know her 'best friend' (30s-40s?F) is her girlfriend? + UPDATE'

My mom (36F) has raised me as a single mom since my piece of shit dad ran off to marry some chick he met on deployment. She has worked very hard and I love her to bits. She's never talked about remarrying and has never even dated to my knowledge.

Two years ago she met Kim at one of my swim meets and they became instant friends. I found it kind of annoying at first, but the last couple years she's been happier than I ever remember my mom being. They go out to dinner once a week, they talk and text a lot, etc.

Earlier this week I had to fix a software problem on my mom's computer and I opened her browser history and saw a page about having safe sex for two women. All of a sudden everything clicked. My mom working late hours at work a lot recently, my mom hiding her laundry from me, things like that.

I want to tell my mom that I'm really happy for her that she has a girlfriend and she doesn't have to hide it from me, but I feel like going 'Hey mom, I know you're munching Kim's rug' wouldn't be good.

Advice?

tl;dr: Pretty sure my mom's 'best friend' is her girlfriend, how do I tell her I know and she doesn't need to hide?

Update:

There were way more replies than I could keep up with! For the day off today we decided to order dinner for takeout, and when my mom told me her order I asked her if I should put in an order for Kim as well.

My mom asked why I thought Kim would be eating dinner with us, so I told her they hang out so much Kim is practically her girlfriend and I'm really happy for them both and they don't need to hide it from me.

Turns out I was right about my mom dating a woman but wrong about it being Kim. About the same time she met Kim, she also met Jin at a professional conference of hers, and they've been dating for a few months now.

My mom invited Jin over to dinner and it turns out they kept all their mushy stuff on my mom's smartphone, I knew they were friends but turns out half the time my mom's 'gone out to dinner with a friend' lately it's been Jin.

My mom hadn't told me for a few reasons. She grew up in a super conservative area and was kinda forced to marry my dad right after high school. She's not sure if she's bi or gay or has an exception or what, she's still working through a lot of stuff about how she feels and what she wants to call herself.

She also didn't want to tell me until I turned 18 because she's pretty sure my grandparents and aunts and uncles will disown her when she comes out, and make me choose. My mom wanted to wait until I was legally an adult and could make my own decisions about my relationships with my family.

I hugged my mom and told her I don't give a shit what some hateful, miserable antediluvian cretins yammering from the primordial ooze say, she's my mom and she's awesome and I love her and Jin seems like a great woman. I told her that if the crabs in the bucket want to pull her back down, make crab cakes with rainbow sprinkles.

Jin laughed her head off but my mom asked me to please not say anything in public. She's working through personal shit and she'll come out in public when she's ready, so I promised I wouldn't say a word to anyone.

Except to you guys, there was so much awesome advice. Thank you, reddit!

tl;dr: right about my mom dating a woman, wrong about which woman, mom's working through personal stuff before she comes out publicly but knows I love and support her