Yes, and it's NOT an unreasonable boundary!!! This woman almost destroyed your marriage, it is NOT okay for him to be going out in these small groups with her. Either you go with, or he doesn't go.
I know we get caught up in trying to be "reasonable, trusting" spouses, but I'll say it again, this is NOT an unreasonable boundary. She shouldn't be calling him and he shouldn't be seeing her outside of work. Period. And even if this request WAS "unreasonable", your H should be happy to accomodate it just so that you don't have to worry.
