As for her, I was hoping to be able to go to our HS reunion this year and be on friendly terms with her. But your point is taken. Deleting is only a click away.
Many times, past hurts and betrayals, brought back to life bring back painful emotionals and more feelings of betrayal, and that fact they were again attempting to hide something would be the same.
As far as the HS reunion, sure be friendly, by letting it go now,and be civil and friendly at the reunion In the meantime, maybe look up some other people from HS on facebook and plan to meet up with them at the reunion too, so if you feel uncomfortable, or if she brings him, no worries you have plenty of others to catch up and be friendly with.
As far as telling her about him, why bother seems she's been around enough to have an idea and has dealt with him on her own previously, she'll find out soon enough