I think her telling you immediately should get huge kuddos, that's a major step!
Since Mr Lucky worked with OW we had worked out every possible scenario you could even imagine (and I'm imaginative ). He thought I was off my rocker ... except I wasn't. OW tried half of them, once if not twice!
I would encourage your WS not to have ANY conversations with OW if at all possible & certainly nothing personal - ever.
Talking about encounters with OW will most likely trigger you & create some anger, however, the most important thing is to create a safe place for her to tell you of these encounters.
Hopefully your WS can do her best to avoid OW at all costs. Now that OW's mom is sick & the very real possibility for her to be around is pretty high ... encourage your WS to head the other direction if she sees OW heading her way, and to avoid any and all potential contact. It's likely OW might take today's encounter to mean that your WS & she can now be "friends".
Be very glad she did tell you about this, and thank her.