How do you end your friendship with a good bestfriend ....

I’ve been friends with my bestfriend for 5 years now. I don’t have many friends because nobody is ever an actual good friend. She’s helped me by staying with pets when I’m out of town, lend me money when I didn’t have any, cried when I’ve cried, we even were pregnant together, I know I could ask her for anything and she’d be there for me. She’s genuinely a really good friend whom I love very much & would do anything for... with all that being said. Me and her slept together one time. When we decided to sleep together it was friendly. When I look at her I never think back to that night or ever miss her and I’m not one bit attracted to her. It just happened and would never happen again. Now I had a baby with my boyfriend who knew this, he at first never said it was a problem my friendship with her. Till he saw a picture she posted of us kissing for New Years 2017. He is upset that I’m still her friend and wants me to understand why he doesn’t approve. Which I understand completely and if it was ever the other way around I wouldn’t approve and would be very hurt. My friend is constantly texting me asking how my baby is (her babies (twins) died ) always buys him clothes and is always asking to see him and hang out with her. She says I’m her only true friend... which I know I am. I know I have to end this friendship. It needed to end a year ago when me and my boyfriend started dating. I want to be honest with her because that’s how much I value her true friendship but also I know she’s going to think I’m dumb for letting a man make that decision. But she wouldn’t understand, she’s in an open relationship and basically runs her relationship and does whatever she wants. She wasnt raised how I was so I see it as respecting my relationship and shell see it as me being dumb and letting a guy get in between our relationship. I’m really heart broken but this needs to be done ASAP. I can’t keep ignoring her and leaving her on read when she tries talking to me. I don’t know what to do. This is so hard. How do I tell her or do I even tell her ? Do I just block her off everything ? I couldn’t do that anyways because my sister buys weed from her and they work together. She literally has my location on my phone has had it for 2 years now. How do I stop that !! How do I do anything. I’m so lost and I don’t want to hurt her. I don’t know what do to man. I just know that friendship needs to end ASAP :(

Update : I need to be more honest since I’m getting put down about the situation. I said one time because the whole sleeping thing wasn’t a problem. But we had 3 sums too. He knows about them too. I slept with her alone twice. 3sum twice. As a man I understand him. As a bestfriend who always did right by me I don’t want to loose her friendship but I feel it needs to be done even if it didn’t continue to happen.