Lots of forms want a middle initial. She usually uses her maiden name. But sometimes they don’t like that because it’s not her legal name.

It’s not. I don’t have a middle name and it’s never been a hassle. You either leave it blank, put a dash or write “NMI” for “no middle initial”. You will see the latter on some US government forms.

Lots of forms require a middle initial.

https://wookware.org/name.html

Here’s a person who finds extra hassle.