Best friend (22F) suddenly said she doesn’t like my other friend (20F) after a beach day because she thinks she was staring at her boyfriend – how do I handle this?

A couple of days ago my best friend 22F (Bella) told me that during a recent beach day our friend 20F (Milly) had been staring at Bella’s boyfriend the whole time.
Bella 22F and I have been friends for about 3 years. She is a very direct, no bullshit, confrontational person. In the last year she separated from a long-term boyfriend and has been dating her new boyfriend since March. They seem to be doing great.

Milly 20F and I were super close in middle school attached at the hip, sleepovers, telling each other everything. We drifted apart after freshman year with no drama, then reconnected last year after we both lost a mutual friend from middle school. Over the last month we’ve become really close again.

I planned a beach day for the weekend and invited Bella, Milly, and another friend (Sam). Bella was over an hour late, showed up after we had already set up at the beach, and had clearly been drinking beforehand. However, she was friendly to everyone, including Milly. She laughed at Milly’s comments, talked with her, and from my perspective they seemed to be getting along well the whole day.

A few days later, I was on the phone with Bella (she had been drinking again) and casually mentioned something about Milly. She suddenly said “honestly I really don’t fucking like Milly.” She claimed Milly was staring at her boyfriend the entire time at the beach, gave her a dirty look when she first arrived, and that she almost said something to her. She even started insulting Milly’s appearance and body. This surprised me because Bella is usually very confrontational in the moment, not days or weeks later when something bothers her.

Nothing I observed that day matched what Bella described. Milly is shy and introverted, and I didn’t notice any staring or weird behavior. I’m really confused about the whole situation and not sure how to proceed with these two friendships. Any advice?