My mum want applying for a fiancé(e) visa to the U.S. My dad left long ago, now my mum found love. What should expect?

Your mom can apply for a K-1 fiancé(e) visa. She’ll need proof of her relationship, meet income requirements, and prepare for an interview. After marriage, she must apply for a green card.

She'll need a k1 visa and some of the stuff Linh mentioned

Expect an interview after applying for the k1 visa? Also there are income requirements she'll need to meet.