Without the sashing as a border the quilt would look like this. I don't think it looks bad. Do you? I do not have cornerstones. All of the sashing and binding would be black if that matters.

Not my quilt or photo, taken from the internet.

The gal I am using says 4 inches bigger on her web. I am hoping that means 4 inches total and not on each side. I will check with her on Tuesday when she opens back up. Mom has her hands full with moving recently and other issues, I would really like for her to not have to spend $100+ on more minky.

Remove the sashing. Depending on how wide the sashing is you may gain a few inches. Most tee shirt quilts I've seen do not have sashing. You need 4" all around the quilt for the long arm

This is one I made.

T-Shirt Quilts & Blankets | Memory Quilts Made from T-Shirts | Project Repat - Project Repat T Shirt Quilts

I've made a lot of quilts, including a.few t-shirt quilts. I always cut my backing with a good 4" margin on all sides. The top of the quilt will MOVE when you or a longarm quilter is quilting it. And you need that extra space on the backing fabric to allow for this. Is your friend wed to the idea of a minky backing? If not, look into "extra wide quilt backing" fabric available in many places, including Joann's. You can also seam fabric to create the right size -but I'd hesitate to do that with minky due to the thickness.

@Bestyears Yes , she wants minky to make it or more of a blanket or throw.

Aah, I see. Okay, then yes, let's see what the longarm quilter says, though I'm guessing it is a 4-inch margin on all sides. You've already stitched the blocks together with the sashing between each block similar to the internet photo you included? If so, you could trim all that sashing off and just stitch the t-shirts together as jck suggests. I've made many quilts in that style and they look great -though, of course, they won't 'match' the others if they have sashing around each block. If you do decide to buy more Minky, Joann's almost always has a 50% off coupon on their app.

This is why we always use flat sheets for the backing that way no seam. Always plan to put a boarder around the top squares, it goes last. It's width dependent on how the top/bottom match up. Measure twice!! Can you add the same front boarder to the back? One thing we learned after the first memory quilt we made for my mom's 25th wedding anniversary (we were all in high school) was you do not need any middle batting. These quilts with their front and back and sizing get plenty thick and warm!!