Terrific job on the "fix". Even more terrific is your friend. And I can COMPLETELY understand your immediate nausea when you discovered the error!! I use "dry erase" markers on plexiglass when I'm "testing" a quilting idea, but I also have a Sharpie that is almost identical. I try to be VERY aware of which one I'm using, and also try to NOT have either of them anywhere close once I start quilting. But, as Forrest Gump said after stepping in dog-doo - "It happens."
Your friend is a REAL friend. Hugs to you both for knowing what is REALLY the most important - your friendship.