Looking for advice about a hedge

I have a customer that just moved into the area. I mowed their lawn twice. They say they mow with a reel mower and the grass was too tall when they first moved in to cut with a reel mower. However, the property owner asked me about cuttting back this hedge. Like hacking it back to about 5'. The hedge is currently about 11' tall. First off I'm a bit hesitant to cut that much off. I was always told the rule of thumb is don't cut off more than a 1/3 of the green. Second, my hedge trimmers are...

Looking for advice about a hedge