i am assuming the problem is rubbing and the soreness is along her thighs up to her privates? If this is the case then without a doubt, the problem is that she is sitting.
I am 5'1 115 lbs. I ride a DRZ 200 with a stock seat. (Stiff wide seat). When I am on the road, (sitting) it takes no more than 30 minutes before I am chaffed. I can ride for 5 hours offroad out of the seat -no chaffing.
Here is my recomendation: Find a nice flat, straight dirt road and ask her to stand up on the pegs for the entire length of the road. If she is comfortable doing this and enjoys it, find a nice mellow corner and teach her how to transition from being up on the pegs to shifiting her weight down and to the front of the bike while making a corner. Now she has the skills to have fun and not be sore.
My next recomendation is if she is willing and enjoys the above, make an investment in both of your happiness and get her a real bike that she may actually be able to build some skills with.
If she can't or won't stand up, then don't make the investment.
-Go to a running shoe store and ask for a product that you can use for chaffing ( endurance runners use it for underarm and thigh rubbing) this may help. Also, she may think you are being kinky but I recomend NOT wearing underwear - The seam and elastic probably isn't helping.
Best of luck