Bree123 has great advice for you. My first 2 quilts where t-shirt quilts for my kids before I learned anything about quilting--and it shows! But I've made about 8 now for clients. I definitely suggest you stabilize with a lightweight fusible on the back of all the shirts--if you are using the slinky poly ones that have built in holes (like football jerseys) the fusible will show through--but it's not too bad once quilted. You want to first cut the logos with about 2" extra, apply the fusible (all those plastic type logos are a no-no to iron---they will melt all over--so if it's large you really can't use it without gluing it down first) to the back and then you'll need to re-cut to the size you want--leave a 1/2" seam as the knit still seems to want to curl and move as you sew.
I highly recommend having some type of quilter cotton sashing between blocks as it helps stabilize the whole quilt.
While I have made very large ones, most people really are happiest with a lap or twin size--it probably would not use every single t or jersey, but suggest she pick one to represent each sport or age instead. They are very heavy and not cheap to make, despite using the t-shirts & jerseys.
I quilt on a long arm & recommend you get I long armed. But if you plan to do it on you DSM, baste it super well (lots of weight shifting around). Depending on the size of the shirt logos and the pictures, you'll need to quilt through the larger ones--but again those darn vinyl logos are a no-no---they tend to grab the needle and hold on, and it there are any problems, it will leave a big hole in the logo. Good luck!