It is too beautiful a quilt to "chuck" it - rewash in cold water and spin dry it - hang it out on the line - stretching it as you go - I'll bet you find it stretches to the shape you had - the quilting that has been done, should have held it pretty well, and besides, you can iron it out using some sizing, and I'll bet it will turn out beautiful... and you needn't say anything to your m-i-l! She did not know what size it was intended to be, like one other lady here said, so just go with what you have - and it'll turn out beautiful... I had a throw that did that - and I just stretched it, and ironed it after dry and using the sizing gives it the "new" look and it was fine..
Always remember to use cold or cool water when washing quilts regardless... it helps with not shrinking as well as colors fading.. and I use vinegar in the water when I do pre-wash IF I prewash. Helps hold the color.
good luck deema - you'll be fine! iron front and back - a lot of work, but it will look fine.