Is past behavior with another person a good indicator of future behavior with you?

Me and my therapist disagree on this. I say it is, she says its not. I’ll give an example.

My friends R and L are very close, so sometimes R tells L about his issues. L is happy to help, but L often comes to me and vents about R’s problems. Not because she doesn’t want to help, but just because it’s something they’ve talked about a lot and L wants R to be over. I’ve been going through some issues myself and I feel like talking to L would help, but I worry she would go to R and vent the same way she does to me, and that would probably cause issues with R.

My therapist thinks my fears are unfounded because that’s their relationship and not mine and L’s, but I feel like if she’s done it in the past, there’s no reason she won’t do it again.