How many through holes are there? What will the customer do with the quilt after the repair is made, use it or display it? What condition is the rest of the fabric, really fragile or OK?
I would consider a whole new back to support the quilt, and probably tie with matching floss. you can insert pieces of batting and re piece the holes or applique something over the hole, as long as the surrounding fabric will support the applique. If it's really fragile, the applique could put additional stress on it.
Wost case, could the quilt be cut to a smaller size throw or maybe smaller pieces could be framed?
I also want to know how old the customer is if the blocks could be 100!