Her visa will only be issued in the name on her passport, so you must use that name when you apply for the visa.
In terms of changing the name on her passport, that's completely up to her. She can keep her passport and visa in her maiden name if she wishes, and can just use her married name for everything else in the UK.
So, if she is happy to have her visa issued in her maiden name then she doesn't need to change her name on her passport. However, she will need to remember to always book travel in her maiden name as the name on her ticket must match her passport and visa.
If she would prefer her visa to be issued in her married name, then you'll need to change the name on her passport first, before applying for the visa.
If there is any chance she may want to change her name on her passport in the next 2.5 years, I would change it now, before she gets the visa... Because it's a pain to change it on the visa after it has been granted (it costs £223 to do it later).
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