Probably just because she wanted to.
Here's my name history.
GoofyFirstName NormalMiddleName UnpronounceableMaidenName
I've always gone by NormalMiddleName but always have to fill out forms with GoofyFirstName, so that's my "official" name and #1 I hate it and #2 most people mispronounce it.
With my first marriage, I became:
GoofyFirstName NormalMiddleName NormalLastName.
Divorced, and I went back to
GoofyFirstName NormalMiddleName UnpronounceableMaidenName
Second marriage, I became:
GoofyFirstName NormalMiddleName FrenchLastName
Had a child. Got divorced. Got the idea in my head that my son would be "confused" if he grew up with a different last name than me, and I didn't want to go back to the UnpronounceableMaidenName anyway, so I kept the FrenchLastName for about ten years.
More and more, I hated being linked to the exhusband and all the b*llsh*t he insisted on perpetuating on me and my son. After thinking about it long and hard, I went to court to change my name to:
NormalMiddle(NowFirst)Name Initial NormalAndRandomLastName.
I picked my own last name at random, based on what was easy to spell and pronounce, sounded good with my first name (which had been my middle name, but has always been the name I've used), and wasn't too unusual or "freaky." It has no meaning or significance other that it's easy and normal and has no unpleasant emotional baggage.
My GoofyFirstName was a combination name given to me in honor of both of my deceased grandmothers, and although I hate it, I didn't want to *totally* break my parents' hearts, so I selected the *initial* of GoofyFirstName as my new middle name.
As far as my parents and other relatives know, I just flipped the names so that my actual name, the name I go by, is now my legal first name. I'm 40, so it's not that I particularly feel the need to hide it, I just don't want to unnecessarily upset them by letting them know that their mothers' names have been abandoned.
At the same time, my relationship with them is strained enough that I don't feel any connection to them, and don't feel that their last name is in any way part of my identity, as my NowFirstName is. Even if my maiden name had been something vanilla like "Jones" I would still have chosen my own last name.
SO, here you have an example of emotional issues, independence and person-hood issues, and convenience/practicality. Any of those, all of those, or something completely different could be the reason for your acquaintance's choice.