If you want any flowers this year, wait a few weeks until after they flower. Forsythia blooms have already set for the season. Best to prune these with loppers. Take the old canes right to the ground and rejuvenate the rest. What you customer is suggesting, is not the proper way to prune Forsythia, but if that's what they demand, loppers will be quickest and easiest, especially if you have a helper removing and bundling what your cutting.
Kirk
Kirk
