personally, I have to applaud you on this generous fix. Just how much did you trim off the inside border? I bet I wouldn't even notice a few missing inches but then I don't make quilts to fit on beds perfectly like some people do.
Yes it is always best to call but I would be thankful to you for being diligent in your work and doing your best to remedy an unruly fabric.
Just be honest (" I didn't know what I was thinking by not calling you before I adjusted the material. It just wasn't playing nice.") Keep it simple and offer her a free panto design on a future quilt. Let us know how it goes for you.
peace