Should I (20F) tell her (20F) he’s (20M) still calling me? I don’t want to lose her again

This all started five years ago, back when I was in my first year of high school. We were all young, immature, and a little naive.

I had a crush on my male best friend (let’s call him M). When I told my female best friend (K), she seemed happy for me… only to start dating him shortly after. It hurt, but I stayed friends with both of them.

They were together for about two years, and when they finally broke up, things got really messy. K blamed me for the breakup, even though I had done nothing wrong — in fact, I always tried to help them work through their problems. She even spread rumors about me being the reason for their fights. I cut contact with her after that.

Fast forward about a year and a half after their “breakup,” M started flirting with me. I believed he was genuine. K and I weren’t friends anymore, and I assumed she had moved on. M and I flirted for over a year, and he even talked about marriage and a future together.

Eventually, I started running into K again — we still share mutual friends. I didn’t want to keep holding on to old resentment, so I reached out. Surprisingly, she was really happy to hear from me. We talked, cleared the air, and she apologized. She told me love had blinded her and that she regretted how she treated me. It felt good to have her back in my life.

Then, six months later, I found out the truth: they had never broken up. The whole time M was flirting with me, he was still with her — and telling her that I was the one trying to come between them.

I was shocked. I called K, told her everything, showed her the screenshots, and blocked M on everything.

Since then, K has been incredibly kind to me. She even threw me a surprise birthday celebration. She told me she never wants to lose me again because of him — and I feel the same.

But now… the problem is, he’s still calling me. On the night of my birthday, he tried to call. I didn’t answer. He’s called before using an unknown number, but I recognized his voice. Just yesterday, I got repeated calls from another unknown number — same time of night, same pattern. I’m sure it’s him.

Now I’m torn:
Should I tell K that he’s still trying to contact me?
She’s still in love with him, and I know she’ll probably forgive him no matter what…
But what if she finds out later and blames me for keeping it from her?

I really don’t want to lose her again.

TL;DR:
My best friend (K) dated my crush (M), blamed me for their breakup, then apologized and we reconciled. Later I found out they never broke up and M was lying to both of us. I cut him off and rebuilt my friendship with K. Now he keeps calling me from unknown numbers, and I’m unsure if I should tell her or stay silent.