They both should survive the whack. Spread 1-2" compost as mulch & water weekly if no rain.

Snowball bush will likely produce a lot of upright shoots as they're sensitive to pruning too much. Spray off the black aphids that appear on the new growth. They seem to get less aphids in part sun & moist soil than when I had them in full sun. Don't fertilize other than the compost as that will promote even more growth, which you don't want.

Lilac won't bloom next year. To keep it tidy & get yearly flowers you can prune 1/3 of the branches back to stem each year.

Once they're dormant in fall think about moving one of them away to avoid crowding.

I've moved our snowball bushes several times & usually in early spring. They've always survived even though they were coming out of dormancy then. I just moved one again this spring & think they're in permanent places now. If not, I know I can move them again despite what I've read in books.

marynj - I hope you weren't too hard on your brother, but I cand feel your angst. Different types of hydrangeas are pruned differently. There are many good sites. Here's one Good luck.
-Sprout

Here is a link that might be useful: Pruning hydrangeas

Oops, Sprout, I think marynj has an old fashioned snowball bush, not a hydrangea. Or I could be wrong, maybe they are in the hydrangea family?

I prune my ancient snowball bush after flowering because it gets so much new growth on it, but I've never cut back the woody trunks. Like corrine says, tho, I bet you'll see new green shoots up from around the bottom. Not sure about off the wood?

I wasn't TOO hard on him, sprout! LOL My brother was trying to help out and I don't think he intended to kill anything.

This is probably a blessing in disguise. The snowball bush was getting pretty gangly-looking, although it was still blooming. I would really hate to see that die. I'm not quite as concerned about the lilacs because we have so many and he didn't cut them all back.

Thank you, folks!

The snowball is probably a viburnum, not a hydrangea. They don't object much to pruning as long as it is done soon after flowering. Yours might not bloom next year, but it should be a better, more compact bush for it.