This is not a great project for a beginner. I tried t-shirt quilts once & swore them off for life.
But if both of you have more patience than average & a few extra t-shirts in case something goes wrong (it often does), then here are the other tips I can share:
1) Pellon 911FF is great provided that the tee's are 100% cotton. This is very important. Pellon products are designed for 100% cotton & will not bond properly to synthetic fibers or blends. I like it better than the 906F because it gives just a touch more stability to the knits, making them piece together a bit nicer, imho, with quilter's cotton.
2) Fusible bonds better when pressed against a hard surface. I covered a piece of particle board shelving with paper grocery bags (tape with masking tape or painters tape, NOT plastic packing tape or Scotch tape as those will melt!).
3) Strongly recommend using a pressing cloth. It will keep you from pressing too long & melting the vinyl decals.
Is there a reason why you don't want to use batting? I'd go with a cotton backing & low-loft batting vs flannel without batting every time. I wouldn't recommend fleece for a beginner project. It shifts a lot. If she plans to hang this quilt on the wall, I'd recommend a low-loft polyester batting like Poly-Fil. If she plans to use it either as a picnic quilt or a lightweight bed quilt, I'd either use Warm & Natural 88% cotton batting (12% synthetic materials for scrim), or Quilter's Dream Request 100% cotton batting. She could do Poly-Fil for a bed quilt, but polyester doesn't breathe so often people sweat under it and either get too hot or chilled depending on room temperature. The nicest option is wool -- lightweight, warm in the winter, cool in the summer. It does cost more, but Hobb's Heirloom (washable) wool batting is super luxurious and so easy to quilt.
Finally, she certainly can stick to SITD, but if she plans to use it as a bed quilt (which I'm assuming, since she asked for a Twin), I would really recommend reconsidering if maybe you could help her out some once she finishes with SITD so she can add additional quilting. The amount of time it takes to stabilize all those t-shirts is about 10x longer than one might think. It look me twice as long to apply the stabilizer as it does to needle turn applique 10,000 stitches at 1/14" each -- no exaggeration -- and I can charge 8x more for my needlework. After such an enormous time investment, I'd think twice about whether I'd be okay with the goofy thing starting to come apart in 4 years. I'd really recommend at least doing crosshatching or a large meander to fill in the space between decals so there's no more than 4" between quilting lines. I suppose you could even just quilt a box around the motifs & use that for travel stitching. If she can do SITD, she could follow a marked line. Just grab a Crayola Ultra Clean marker -- or the marking pen of your choice -- and a ruler & draw the lines starting at the center out. Pretty easy even for most beginners to follow.