Here's how I did it. I took something 18g and inserted it and left it in for a few hours. Then I found something 16g and wiggled that in with some lube. After several more hours, I held hot water in my mouth for several minutes. I took a lubed 14g taper and used it to insert a barbell. It was somewhat uncomfortable, and my tongue was a little swollen the next day. It wasn't anything compared to the swelling after piercing. After a few days the small bit of soreness and swelling went away. I do not recommend this, I took a risk stretching from 18g to 14g in one night. I was cautious and would have stopped at any pain, but sometimes you don't realize you've injured yourself until later. I was lucky. The best course would probably be to either use tapers and try to leave barbells in for a few days minimum, or use teflon tape to stretch. Teflon may not be practical in small sizes and damp piercings, however. And small gauge jewelry in a floppy tongue may be uncomfortable and has its own risks, there's a reason they aren't usually pierced smaller than 14g. If she doesn't want to try it, she can just repierce it, which would probably be easiest and possibly safest.