Brushing it off is worrying.
I once raised a similar issue with my partner when we first met. He had had a lot of female friends, and before me, they regularly used to go to the pub in large groups.
When we still lived near this group, I raised an issue I had with a particular girl as I'd kept being told she had "liked" him for years. Also, at a wedding, she "drunkenly" sat in his lap in front of me.
When I raised it at home, he immediately got annoyed, frowned, said there was absolutely no interest on his end, etc. Totally shut me down. I totally believed him, and yes, I was just being paranoid. That's the sort of response you want.
In my vast experience, brushing it off can be a worry. Not always. But given that she's writing him that sort of note, and I'd say she has feelings for him, I'd be very worried, OP.