Nexus = your activity is treated the same as if you are there, such as, I work from Montana to provide a consulting service using the internet to Texas or CT and they want me to pay Sales Tax on the service rendered, the same as a local provider would be doing.

Wayfair = the people that advertise on TV for online ordering and delivery of household goods, including headboards with bed bugs. They got sued by South Dakota for not collecting sales taxes on sales made to people in that State. Wayfair is the settlement that opened up the world wide web to Nexus for sales taxes "across State Lines" for purchases from online sellers.

"It is possible the Ohio travel is related to a portion of the income reported on the 1099 from Ohio, but not 100% certain."

Your Client knows. No one is just working blindly not knowing who hired them for what or why they just traveled to OH or not knowing who paid them. Sheesh; hold that client responsible. And it's not just 1099 details; it is All Revenue that needs to be part of tax reporting. You know this. Tell the client to give Details.

"Let's take CA. I used to be a tax prep in that state, and I know they like to tax everything."

Try this one: A CA lottery winner moved to MT and all their lottery income is taxed here.

"But there's no 1099 from CA. How would FTB know she earned money there?"

Because a company with a CA tax return that filed information returns (1099) with their own CA address has reported there are payments made to someone not having a CA address? I like to use the word "discoverable" as in, if there is any sort of paper trail, that is Discoverable, if someone starts looking.

"If she's required to pay, how much should she pay?"

On what she reports, for what qualifies.

"Should it be based on the income she received from CA (which is technically $0, according to the 1099-MISC) or the amount of time spent in CA?"

Well, this just got confusing. You stated there is no Income, even though she worked while in CA? But, no income means no work.

And remember, even if she makes $2,000,000 as long as no one party pays $600 or more, there is never any 1099-M (NEC) issued. So I would drop the presence of 1099 from the discussion of proper reporting and filing of taxes, since it doesn't apply to what you, a professional tax preparer, are supposed to be doing.