In reply to GrahamD:
What mileage was she doing before and what has she increased it to? Could it be simple 'buttache' from more time in the saddle than she's used to? I feel a bit bruised after being out yesterday on a 70 mile ride despite having done 100+ mile days on the same set up last year - my bum's just forgotten what it's like to be on a bike for long periods.
What kind of saddle has she got? I have a fairly hard women's specific saddle - more painful to get used to when you're upping your mileage but ultimately more comfortable once you're used to it. Also, does she have good comfortable padded shorts?
That said, a bike fit is well worth the money if she's not comfortable. I got one last year and it felt like I'd bought a new bike! Saddle height, angle and position all make a difference, as does bar height, stem length etc. Being in the wrong position on the bike has a knock on effect all over your body so if she's keen to spend more time in the saddle it's worth looking into.