Your husband loves you, and he respects you, which he proved by deleting the ex-wife from FB when you expressed your concerns. He has no need of staying connected to her. Did he tell you about the message or did you see it on his FB? I guess that doesn't matter... I would just tell him to tell her no. If he keeps declining/ignoring her requests she'll get the picture. It may simply be that she knows your husband is knowledgeable about something that would be useful to her sister, and she doesn't feel weird about asking him for advice. If he can help her, why not? You say that they haven't been in touch for over 17 years and have both moved on, so unless there is some other reason you distrust her or your husband, I don't really see the harm in it. Also, as an aside, my husband and I both have FB pages. For me, it seems like a lot of people "friend" anyone and everyone they have ever known in their whole life, even if they never intend to go any further than that initial contact. It would be different if she was sending messages to him frequently, or commenting on all his posts. But if this one simple request is all she's ever done besides requesting he "friend" her in the first place, it's probably pretty innocent, and if it were me I would treat it that way until I could prove it otherwise. No matter what though, your husband should respect your wishes. I'm sure he's not the only one she could go to for advice, so if it truly makes you uneasy, just tell him to say no and that's that.