So great that the tech rep helped you with the tongue!
Now to fix the slipping heel because they aren't narrow enough...
If the bottom of her heel is too narrow for the boot, a heat mold won't completely fix the bottom - though if the rest of the heel is made snug enough it would help. Making a new insole that snugs everything up probably will.
I assume that she is old enough, senstive enough, and self-aware enough to be able to feel places the boot fits her foot loose and tight, and places where she needs more or less support.
1. Do the heat mold. It may not be perfect, but it will almost certainly help.
2. Buy material for a new insole. I like to use closed cell carpet foam, because it is nice and warm, though an open cell caroet foam might be better if her feet tend to be hot and sweaty inside the boot. Buy extra, so so you can do this again a couple times, if everything doesn't come out perfect. Carpet foam is cheap, and it sounds like you/she cares a lot. People have used other types of foam, like upholstery foam and craft foam. I figure carpet foam is reasonably skin safe, but still has anti-mold and antibacterial chemicals.
3. Mark the line on the insole that the insole bends upwards while inside the boot (near the ball of the foot).
4. Have her figure out any places where she wants extra support (e.g., if the arch of her foot is higher than the arch of the boot with the insole in it, or if the current fit causes that side of the foot to bear more weight than the other), or less support (e.g., if the arch of her foot is lower, or that side bears less weight).
5. Remove the insole.
6. Trace the insole outline onto the material for a new insole.
7. Trace her foot inside that material. Position the foot so the ball of her foot really is on the line from step 3, because that is where human feet are able to bend without pain and inflammation. In many respects, that is the most important part of a perfect fit. Side-to-side, position her foot so it is centered within the outline.
8. Cut the material. You more or less want to match the outline of the old insole. BUT, in places where the old insole is wider or longer (but not along side of or in front of the toes, because you want extra space there), leave a little extra material (maybe about the amount by which it is too wide or long? Or a little more, to be safe, and you will trim it for comfort later). Once put in the boot, that extra material will come up alongside or behind the foot, and make it perfectly snug, near the bottom.
9. Put the new insole in the boot.
10. Put the foot in the boot. Remember to push down very firmly on the heel, even to pound a bit on the back, so the heel goes all the way down. People whose heels are not the narrowest part of the foot, she may have to do this every time she puts on the boot - though if her heel is narrower than the rest of the foot, she may not need to do it.
11. The carpet foam will be thicker than the original insole, so the foot will be much too tight.
12. Cut the underside of the foam in places with a scissors to make the fit perfect. You more or less want to duplicate the thickness of the original insole, but leave it thickest where she wants more support, thinest where she wants less, as per step 4. You don't need to match what she felt in step 4 exactly - you can make it even better now that she can feel the improved fit. Remember that you probably don't want a snug fit along-side or or front of the toes! - for the most part, that is bad for feet.
13. Put the foot in again. Cut to adjust again, and iterate until the fit is completely perfect.
14. If everything wasn't perfect, and you cut too much somewhere, start over with a new piece of foam, and try again.
15. If there are other places on the foot with less pressure than others, add moleskin to the side of the boot there.
As long as the boot wasn't too tight anywhere (or can be made properly snug by cutting the insole, or stretching the boot), you can use this technique to make almost any closed shoe or boot fit perfectly. I think they should teach it to everyone.
I hope that was all clear.