The first batch can take longer, but after that, the time should be the same as non-stone time.

Also, baking them on the bar pan is going to yield different results than baking them on a flat stone.

 

Is the dough cold or just mixed. That makes a difference too. Is this a new recipe for her or a tried and true?

Like others said the first batch takes longer even with a preheated stone.

Also maybee her oven has begun to go flakey... it can happen

 

I e-mailed the test kitchens once because I was really struggling with baking cookies on the rectangle stone. I was trying to use parchment paper because some people had recommended it. My cookies were taking almost 20 minutes for the first batch too. They would look done on the outside, but be really undercooked in the middle. This is what test kitchens wrote me:

Because you are using stoneware, you may have to bake a little longer since
stones sometimes take a little longer to get hot in the oven. I wouldn't
waste parchment paper on the cookies though. You don't need it with
stoneware. I only recommend parchment when baking with a metal sheet
pan. Try baking the cookies on the stones without parchment and see how
beautifully they come out on the bottom. They're perfect! This will also
season your stone really well and your cookies, over time will be
excellent when baked on your stone! I love my stones for cookies!

Since then I've used the stones just plain and now it's like they said, the stone is getting seasoned and they don't stick to it and they are coming out really nice. But the baking time is longer for the first batch, and I let them sit on the stone for a few minutes to continue cooking just a bit.

 

I just made cookies a couple weeks ago for the first time on my stone, and the first batch did take me about twice as long, but after that the suggested baking time worked well for me.