A big fat padded saddle is no good for a long ride. If she’s fairly new to commuting that distance, it is gonna hurt to start with, but will get better! Check her seat is high enough, too low and she’ll be putting all her weight on her sit-upon and none on her hands and feet.
If you are in Manchester as your profile suggest, seriously take a look at my link to FREE road skills training.
Narrower, smoother tyres are the simplest way to make a mtb a little easier to ride. I use Marathon Plus, so don’t have to worry (too much) about p***tures.