I'll bet that whole quilt was a beauty and worth trying to mend. Maybe draw a template of the diamond shape and background allowing for your 1/4" seam and "paperpiece" the shape then applique over the holes, adding batting where needed. That is quite a job when the hole goes all the way through. I've done pieces for fronts and if you get lucky with your fabric it is hard to spot where the repairs are done. Good luck finding fabric near what has been used. I hope they really love the quilt because it seems as if it would be not only time consuming but expensive as well. I would show how you are going to repair it beforehand to be sure the owner knows how it is going to look. It will be very satisfying when it is complete.