If two people filed a joint return and one takes all the money and refuses to give any to the other person who signed but didn't have income is that illegal?

That is a legal problem.  You need to see a lawyer.  

there is a provision in the code that you can go from a joint return to married filing separately. There is no way to determine if you would be better off financially if you did this.   consult a tax pro. 

When it comes to tax filing, transitioning from a joint return to married filing separately is a decision that married couples can make. Here’s how it works:

1. Married Filing Separately (MFS): Under this status, each spouse files their own individual tax return instead of combining income and deductions on a single joint return. It’s essential to note the following points:

   • If one spouse itemizes deductions, the other spouse must also itemize.
   • Certain deductions and credits are restricted when filing separately, such as the credit for child and dependent care expenses, earned income credit, adoption credit, education credits, and the deduction for student loan interest.
   • Filing separately is not the same as filing as a single individual; the tax brackets differ.
   • In community property states, both spouses may need to declare their income even when filing separately.

2. Spousal Consent: Generally, you do not need your spouse’s permission to switch from a joint return to married filing separately. 

It is important to note that if the election to switch is done as Mike9241 indicates above that it must be done before the deadline to file the tax return, without extensions. 

After the deadline, the election to file a joint return is irrevocable.