My father has two very overgrown holly bushes. they are now trees!
One is 5 -6 above the roof line. I am making arrangements to cut back these
Bushes. can I do this now, without killing them?
You won't kill them, in fact you can cut them back right down the the stumps to allow all new growth to start. You may not like the looks of the trees if yhou cut them back just a few feet, but regardless, pruning will not kill. If you do this, the best time of year is in the sprink before the new growth emerges.
Ok forget that. Today I picked up my copy of Fine Gardening and lo and behold, there is an entire chapter on pruning hollies. Pick one up if you can, there are different type of holly and you can accurately identify yours from that magazine. It's one of the best mags out there for gardening info! Good luck...