Instagram doesn't work like Facebook. Following doesn't mean you are being followed by the person. Unfollowing is essentially the same as taking them out of your feed on Facebook. You don't see their posts anymore, but they are still able to follow you. It's like Twitter--I don't have to follow you for you to follow me.
If it's between her and your DH, I would talk to DH about it and let him be the one to have her unfollow users that are inappropriate for her. FWIW, Instagram has always made me a little uncomfortable because it's public, too. I don't really even use it, even though the filters are fun. I use the filters and save to my phone instead of posting.