That's it my dear, we must learn to let love lead. I want to start by saying 'Happy birthday to you'. Putting efforts to build each other, is highly dependent on the individuals involved.

Now to the issue at hand

I seem to understand what that mother is doing and it's quite unfortunate and very bad. Is the young lady a student or a graduate? If she is a student, maybe her mum had ugly incidents as a young lady or she has seen people who were distracted by men and she's trying to over protect her daughter without communication.
As parents, our children have the right to know why we give certain advice or act in certain ways. I am not the best mum but you will not believe that my tender children know virtually everything about me. They know when I have money, they know when I am happy, they know the reason why I ask them not to mix up with certain people.

My advice

The young lady should be sure of the young man. Then she should have a heart to heart talk with her mum. Let her counsel her mum and ask her the reason for her behavior. She should let her that her attitude isn't helping issues because she isn't a small girl and her mum will not marry her. Her mum needs serious counselling.
If the young lady is a graduate, she might need to keep a little distance so that she can earn some respect. As long as she is still dependent, her mum might not want to stop the overprotective and overbearing attitude.
I wasn't young at all when I got married but i was sweating and shaking when I went to introduce my husband to my parents. Some of my classmates already had children in primary school and as I speak, some have their kids in secondary school but mine are still very young. I don't want my children to pass through such. I don't want indiscipline and flirting but I know their friends and I guide them. They tell me things that they can't tell others. That's how it should be.
@Tobi, death acts funny at times, picking people who are promising. That is why we must live as if it's our last moment. Make good use of every opportunity, help those whom we can, smile, laugh, cry and advice when we can.
I love you all.