Do you really need the money?
In my opinion, it isn't worth charging. You can never make enough to value your time highly enough, and even undercutting it, friends often feel like the price is too high- because they just have no idea how much effort it takes (and t-shirt quilts are a pain, IMO).
Is there something you could barter with her instead? Does she have a craft maybe?
Otherwise, I'd say about $200 for a full size, if she provides all materials, including thread, needles, interfacing, etc.
If you google t-shirt quilt cost, you will find many companies and see what they charge. I found one that is almost $650 for a full (40 t-shirts) another was $350 for the same size (36 shirts) and another that was $375 for a full but estimates 25 shirts. They are obviously seeking a profit though.