I love brainstorming. So just throwing my thoughts into the mix.
There is quite a selection of dandelion fabric at fabric.com.
With the rose theme, I thought it would be great to have a quilt with 65 rose blocks, however that idea doesn't play very nicely with quilts. So more brainstorming ... You could make 64 blocks (8x8), then make borders with one more block as one cornerstone to make 65.
Another idea would be to make a strip quilt with 65 strips.
Or something random-looking like this ...
https://www.pinterest.com/pin/567735096752362950/
Incorporate 65 squares/rectangles ... with maybe some dandelion fabric for the sashing ...
Wishing you the best with this project in memory of your friend.