I have never attempted an angle that sharp. Whoever offered the pattern should have spelled out how to do that, IMO. I just wanted to add that I think the work up to that point was beautiful and is well worth saving. Just my hunch is that a bias will not be enough to go around those sharp turns. What I would try would be to cut the binding at each corner with enough extra to turn under and fold a miter in it by hand in those spots. Use washable school glue (which is a thick starch) and your iron to anchor the binding where you want it to be and then machine sew. Another option might be to hide the corners with something decorative, such as yo-yos, but those would have to be hand sewn, I think, and some of the puckering extends out beyond the corners. I hope your friend is patient because it will be worth the wait whatever way you try!