Fiancés ex girlfriend following me?

Okay before anybody goes "this is so childish" & or "this question is stupid, you must want attention." Please just give me some advice on it ... SO my fiancés ex gf decided to follow me on Instagram & I followed her back. My husband says that he feels betrayed and that I shouldn't be following her out of loyalty to him, being that she was his worst nightmare. She watches all my insta stories & likes all my shit. Should I unfollow her? It's kinda weird to me too being that I don't even like her and we got into a confrontation one time, besides I don't ever even like any of her post. What are you opinions? Please don't bash me ladies bc as easy as is it to just unfollow and block her, I want you guys opinions! Why would you do if you were me? Thanks!!!!