First of all, don't panic.
Second: Consult a lawyer. Because her situation might not be the same as your situation. I know many courts want to do the 50-50 split because they say it's in the kids' best interest - but it depends on your lawyer and also on what judge you end up with and of course all the details of your situation.
My thinking was exactly like yours. I was terrified of what would happen if the kids had to be with him half the time. And I won't lie to you, it was hard. But it all did work out in the end. And during the time they were with him part of the time, my therapist kept telling me that it is better for the kids to have one healthy home and one unhealthy than only an unhealthy home...
Don't give up yet.