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I too would just do it in strips. Before cutting the diagonal on each strip I would stitch just inside the cutting line to stabilize. Then as you stitch the sections together it is very easy to keep everything stable and lined up properly.
I have drafted the finished block with measurements in inches.
Are you sure that the person wants the stripes to be THAT wide?
If I figured correctly, that would be a (finished size) 17 inch block with a 24 inch diagonal.
Or did she mean 12 inches this way?
That is definitely a different pattern.
I would make half square triangle blocks and then sew and flip the extra colour on the corner. You could make them whatever size you wanted.
There would be no bias edges to worry about!!