Should I say anything to my best friend, or let it be?

I had dated M in my 8th grade year of middle school. Broke up after 3 months because we moved to different highschools. In the present time, I've been friends with L for many years now. He is my best friend. I met him in 7th grade but M and L went to the same high school.

Anyway, L is now engaged to R. However, before L was with R, he was fooling around with M but never had sex with her. I warned L away from M a couple weeks ago when he came back home to visit.

Now... M asked me to stay the night with her last week and I accepted and we ended up fooling around a lot but never had sex.

Should I tell L about the encounter or should I just not say anything about it? I don't like to lie to him, but if we are going by the "bro code" he never really had a claim to my ex. Right? I'd like to hear your thoughts on it
Should I say anything to my best friend, or let it be?
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