If you take out the romantic part of the relationship, then you should have your answer. I'm more comfortable being able to share and just openly talk to the people I care about. If a friend, or a coworker said something like that, or someone told me about it, I wouldn't even question telling my husband or boyfriend. Though I would explain the context, being hearsay and my own feelings on how others seemed to react to it. So if it was a metamour, why would I not?
I often tell hubby or boyfriend something the other said or something that happened to them at work or somewhere, this wouldn't be any different.
It sounds like you are worried about coloring his view of her, I can understand that, but as long as you aren't telling him this is how it happened and just what you were told, then you are just letting him know something you do and will probably affect him in the community you are a part of.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year