Even within the category of sports jerseys there is a lot of variation & it is difficult for us to give the best advice without knowing what kind of jerseys they are. As ManicQuilter2 shared, some jerseys are very loosely woven and have visible holes. Others, like soccer jerseys & most basketball jerseys are a tighter weave, but they are still polyester with vinyl lettering which means that pretty much any of the iron-on webs or interfacings are out because it would melt the vinyl lettering ... and possibly even melt the jersey itself.
If you still want to do a quilt with slick sports jerseys, I'd say you basically have 3 options:
(1) Pellon 910 Sew-in Featherweight interfacing. As long as you plan to quilt fairly close together (2" or so between quilting lines), you should be fine just putting some basting stitches in to hold while you piece together the quilt. This will still allow for a pretty good drape on the quilt, just make sure to follow all directions carefully & to trim the interfacing after you stitch together each row. **Note: be sure to check that the interfacing doesn't show through on any of the dark or brightly colored jerseys. If it does, you may have to dye it before sewing so it matches/coordinates with the jersey.
(2) Purchase a muslin or quilter's cotton fabric that matches or isn't visible through the jerseys to serve as a lining (or buy Prepared for Dye fabric and dye it to match -- not quick or easy, but truly beautiful). Drizzle or dab Washable Glue on the wrong side of the jersey & carefully press together with the wrong side of the lining. The glue will come out in the wash, so you want to be sure again to quilt every 2-3" in every direction to ensure it stays together over time.
(3) If the jersey is a loose knit (visible holes), you could very, very carefully apply Aleene's Fabric Fusion to the back of the jersey and fuse the WRONG side of the jersey to the RIGHT side of the lining. Follow the instructions carefully & be sure to fix any bubbles or wrinkles quickly because once Fabric Fusion dries, the fabric is permanently fused. This option will be slightly stiffer than the others, but I've used it before on quilts & it works quite well. You do not need to quilt fused fabric as closely together as it will not pull apart. I have used either plastic knives or a cheapy offset spatula (can't use in the kitchen ever again so don't use a good one; also be sure to keep a cup of hot water nearby to clean the FF off before it dries) and just worked with the FF as if I were icing a cake very, very thinly.
Other tips:
Regardless of what method you use, be sure to wash the jerseys and any fabrics you use before you get started. If you used all new fabrics, you can sometimes skip the pre-wash, but since you are doing a mix-and-match and the jerseys have already been washed by their owner, you will need to wash everything except any stabilizer/interfacing/web products.
I agree with MQ2 that, if possible, doing a quilt will all knit fabrics (t-shirts) would be a whole lot easier. You can use Pellon 911FF with knits & just iron it right onto the wrong side of the t-shirt (after you cut out the piece you want).
Be sure to add an extra 1+" to the length & the width before you cut. You will need 1/2" of that for your seam (1/4" on each side) and the extra 1/2" will allow you a little give after you apply the interfacing so when you make your final cut, it will be square.
Also, I would really recommend putting some sashing with quilter's cotton between your jerseys & photo fabrics. It will help transition between the different fabric types and add a bit of give to the quilt top to make it more durable. Depending on what size you want the quilt to be, sashing can be as narrow as 1" finished (1.5" unfinished).
Good luck with your first quilt! Be sure to give yourself plenty of time & do consider asking your friend to at least help with the cost of materials -- interfacing/lining, thread, batting, backing fabric, binding fabric, fusible photo sheets/Bubble Jet Set. All those costs really add up quickly and you're already spending a good amount of money on your machine, pins, needles, rotary blades, printer ink & the like, plus your time. Based on my own experience, it is not unreasonable to expect to spend 50-100 hours on this project if you plan to do the quilting yourself. Backing the jerseys & either fusing or setting the photo blocks takes much more time and attention than most beginners ever imagine.
Best wishes on the project! What a treasure it will be for your friend's family.