A lot of people simply don't have a dominant hand. They're typically grouped with the people for the hand they write with, but that's a pretty incomplete picture. I suspect your friend who "switched" never had a dominant hand in the first place.

I'm one of those folks. I write left handed, but other stuff tends to be mostly, but not completely, right handed. This is mainly since there's a small advantage to right handedness (baseball gloves fit, mouses are there, guitars are right handed by default, etc.). There's a lot of randomness: I eat right handed, except when using chopsticks (left-handed), including long cooking chopsticks, but tongs (which have a similar purpose) are with the right hand.

Assumed unrelated, but responding to a different part of your post: I can also read and write in mirror image with little loss of speed. I've never thought of that being particularly special. I just started randomly taking notes in mirror image in high school out of boredom (and right-to-left is slightly more comfortable for lefties).