While now is not the best time to cut them, I suspect they'll be okay simply because forsythia are very forgiving. The danger is that we might have enough warm weather left for new growth to begin which will then die back with the freeze, so she might lose the new growth but overall it'll be fine. She obviously won't have much bloom in the spring but if she's good with that, prune away.

Is there enough space to allow the forsythia bushes to achieve their natural shape, but on a smaller scale? If so, you might try renewal pruning.

The forsythia in my mixed shrub border had gotten out of hand - a good 7-8 feet high probably - so I got out the loppers and removed about one-third of the canes down to the ground. I removed the oldest canes, and any that were poorly placed, crossing, damaged, or otherwise undesirable. The end result was smaller but nicely-shaped shrubs that had a muuuuch more civilized look ... and they bloomed very nicely the following spring.

As Yourpal said, these plants are very forgiving.

One out of my 6 is going through a , I dont know what time of year it is, because it is blooming.
I have heard that they are very hard to kill once they are estab;ished. So I think you will be fine.

It will live, it just won't bloom much (if at all) next spring.

Cutting them down is fine, like everyone says they'll just be green this year and bloom next year. What I'd take a look at is the roots/tap roots. If the only maintenance your mom has ever done with the forsythias is trimming and pruning then you better take a look at the ground around them. I spent the better part of one year digging up 10-15 years worth of unchecked forsythias. I mean DIGGING.

With bandsaws, chainsaws, shovels, lopers, hatchets. Digging deep and going out as far as 8 feet from each plant.

The reason I had to go through such enormous lengths was because the roots/tap roots basically choked every single square inch of earth around each sprig and full shrub. I couldn't plant a thing within ten feet of them and worst of all, because they were so overgrown, they didn't even bloom. There were just long green messy things that went naked every winter. The previous owner's negligent 'gardener' even left a cluster of 5 or 6 plants (who knows how many were there to begin with before tap roots did their job) to grow and grow...it was a 12 foot 'dome' of weeping willowy looking branches...that never bloomed.

it was a house for bunnies and birds and all their bird feathers and poop and pee. On top of that, every branch that touched the ground layered suckered and made a NEW forsythia plant. It looked like a meteorite hit my yard in every place I was able to excavate these nightmarish perennials. Their suckers were popping up in between my crabapple tree and azaela bush roots and under fence posts.

You'd have to pay me a million dollars to plant even ONE more of these nasty things. My neighbor who's shrubs marched over my property line unchecked has a 'fence' of them along her back yard. They've marched about ten feet up her own back yard and she doesn't even know. It also is home to rabbits and groundhogs that make it impossible for her to have veggies or flowering plants.

Hire a tree company to get a backho and take every single one out and replace them with something pretty that flowers and that doesn't send out taproots like the forsythia. All you need to do is leave part of a root-like a dandelion or pokeberry plant and the whole thing regenerates.

Use scotch brooms or shrubby butterfly bushes verbenas.

There was a rootball i finally got out of my yard that was a combo of about 4-5 merging root systems from a forsythia planting from my neighbor's yard. I had at it with my bandsaw, my shovel and finally a chainsaw. When i took it down to size it was still the size of a small hippity hop and i was barely able to lift it into my wheelbarrow to get rid of it. I'm still combating the sprouts from my neighbor's shrubs. She doesn't even realize how infested they were and will continue to be. Every branch hit the gound and suckered. Her gardner just comees by once or twice a year with a gas powered hedge trimmer and shapes it and walks away. Doesn't rake under it doesn't...