Should I tell my best friend her boyfriend is on dating apps?

I was shown screenshots by a mutual friend last week. Her boyfriend of two years is active on Tinder and Bumble. My best friend is crazy about this guy, she has mentioned marriage. I do not know if I should tell her, show her the screenshots, or stay out of it. Last time I got involved in a friend relationship situation I lost the friendship. But if it was me I would want to know. What is the right thing to do here?