Not sure why or how a person's gender should determine how far or how much one can ride.
Plenty of fit healthy females and males riding both long and short distances.
Echo others suggestions about bike fit, rider working up to a distance, comfortable pace, etc.
Also would add- make sure her bike is good quality with reliable components and tires.
A bike she likes. My spouse has been thru several bikes, two she really liked and rode them
a lot. One was a (now vintage) green nashiki mountain bike. She road that till the frame gave
out. Then got a bike every couple years- felt ok at first but just not quite right and she didn't want
to ride as much. Last summer she tried a T2000, kept saying she didn't really want another bike.
On the way home (an hr ride to lbs) she was quiet in the car then all of a sudden said- I want that bike.
An email to the store held the bike till the next weekend. Then we went back, the lbs did a nice job with
fitting. had to wait another week to get in some exchange components. Finally she was riding this bike
which she is comfortable with, likes a lot and rides well. An investment in spousal harmony and happiness.