Should I tell my bestfriend's boyfriend she is HIV-positive?

I am a 29-year-old Cameroonian lady and I have been living with HIV now for 6 years. I met Lum (real name withheld) the lady I now call my bestfriend during one of my many visits to the hospital.

When I met her 3 years ago, she had just found out she was HIV-positive and was very suicidal. I became her unofficial counsellor and made her understand that HIV is not a death sentence and she still has a chance of living a long and happy life.

We became really close after that and when she got better and accepted her status we swore to always be there for one another and never to tell anyone about our status.

Ever since I have known Lum, men have always thrown themselves at her but she has never really shown any interest in any of them. I can't blame the men, Lum is a very sexy lady and takes her antiretroviral drugs religiously. I think most people will find it hard to believe she has HIV even if they saw her test results.

Last year Lum met and fell in love with a young banker based in Douala. The relationship got serious pretty quickly as the guy gave her the keys to his apartment.

Before they started having sex, I advised Lum to tell the guy about her status and she promised she would but never did. Lum's boyfriend, let us just call him Karl (not his real name). Karl told Lum he doesn't like condoms and they have been having unprotected sex for months now.

I begged Lum again to tell the guy but she said she was taking her medication as asked by the doctors and that the risk of infecting Karl was very low. But I advised her to tell him and let him decide for himself if he wants to have unprotected sex or not. Lum still didn't tell Karl.

In July this year, Karl proposed marriage to Lum. He still does not know about her status. Karl treats me like Lum's sister. Most of the times when he is buying gifts for Lum, he buys me something too. I have been giving him indirect hints to ask Lum for an HIV test but he doesn't seem to get it.

They are planing their wedding for this December. I feel like if Karl marries Lum without knowing her status or gets infected by her, I would be as guilty as she is for not telling him. I feel like telling Karl would be betraying Lum which is something I swore I would never to do.

What should I do? I am in a dilemma!