The going rate for making a tshirt quilt in my area is $15 a shirt, plus supplies, plus quilting.
You need to cut each tshirt to a uniform size, let's say 13" which would allow for a 1/2" seam so finish at 12". Another common size is 15" finished. Every tshirt needs to be stabilized with fusible non woven interfacing such as Pellon 911. Actually you would rough cut the shirts, fuse them, then trim to the size needed. It's a big, time consuming job. Those who make these a lot, buy a heat press to use instead of an iron...
Often they have sashing and cornerstones, perhaps 2"-3" wide, then a border of perhaps 5" - 6". They could be set together without sashing of course, but sashing does help to keep the bulky seams away from each other at the corners. Press each seam allowance open to reduce bulk too. Bulk in tshirt quilts is one of the issues to watch out for, you don't want to sew over bulky corners and risk a broken needle. The other is if the interfacing doesn't fuse well, and pulls away... that can create folds and pleats on top when you are quilting it, because the knit tshirts will stretch when quilted.
After the top is pieced, use a light poly batting. These babies are heavy!! So cotton makes them even heavier. I use Quilters Dream Poly when I quilt them. The back can be any nice cotton.
I use a longarm and mostly do a large, simple meander, avoiding all intersections. Just a safeguard for those bulky seams. I also avoid quilting in the most rubbery of decals. They are sticky and grab the needle and thread and can shred/break the thread. On a DSM, I'd use a demin needle for strength, and a poly thread like Dual Duty. You will have to stabilize the quilt well, with safety pins and stitch slowly. You could meander or maybe just do strait line quilting with a walking foot, but not SID, rather find a path that is not going to hit the rubbery parts and avoid those bulky areas... perhaps a square around each image and then lines in the sashing like a grid?
Most tshirt quilts are large, bed size and they are very heavy to maneuver under a DSM. You will have a tall order to get into the center of the quilt with all that quilt in the harp area of your DSM. Perhaps she/you should consider sending it a longarmer, esp if it's more than 9 shirts.
As far a sweatshirts, I think I"d pass on that!! And don't mix tshirts and sweatshirts, it will be nightmare to quilt!
HTH and JMHO
A1 Elite Longarm with IQ