Does it makes sense to buy a women's seat for man's mountain bicycle?

What matters is that the saddle is comfortable for the rider given the rides they're doing.

There are obvious differences between male and female anatomy, which aren't to be neglected (here's some in-depth further reading, though the focus is on long road rides). This can lead to a need for different structure towards the front of the saddle in particular. This is most true if riding in aggressive (tucked) positions, so road racing, time trials etc.

We can't make any assumptions about how sit-bone width relates to hip/pelvis width or sex, and in a more upright position the sit-bones, which always bear most of the weight, are hopefully the only parts experiencing real pressure.

In technical mountain biking, you're not on the saddle much anyway, so it matters still less. Still, the longer end of this, if spent on the sort of trails where you'd sit down, can be really rather uncomfortable.

You don't say how much cycling she's done. If she's coming from very little, some initial discomfort may be expected - try gel-padded shorts/tights and anti friction chamois cream at first. New riders change quite a bit around the weight-bearing parts, and may even lose some fat, i.e. change shape, so rushing into a new saddle may not be a good idea.

My suggestion is that she starts with the stock saddle. It may be absolutely fine. If she's experiencing discomfort, especially numbness or obviously pressure-related discomfort that isn't improved by padded shorts and chamois cream, then it's time to look for a new saddle. It would be sensible at that stage to consider women's saddles, though not exclusively.