Have you tried to make contact between her and the fish? I mean introduce her to him, tell her about how amazing they are, about their quirks, spawning behavior etc...
My mother was indifferent to our pet birds and fish a while ago, but after we got our first male canary and she heard him sing, she had a 180 turn. When he died she was crying more that me and my dad.
Now she loves our new one too (more the male than the female, because the female has the personality of a female betta lol).
I believe your fiance needs to interact a bit with the little guy. Try to convince her to feed him, explain that his fins will heal and will be gorgeous.