This is a personal decision, but you are correct, without knowing what her financial situation is, he could end up being responsible for her tax liability. If they are separated and file separately, then his SS will be 85% taxable. If his SS is $24,000 then $20,400 would be considered taxable income. Even if he files jointly, depending on how much income she has, it could still be up to 85% taxable.
If he would file as MFS, then his standard deduction would be $16,150 since he is over 65, which would mean he would have taxable income of $4,250. That amount would be taxed at 10%.
You are under no obligation to have his taxes filed jointly with her. Whether married and living together or separated, each individual always has the choice to file separately if it is in their best interest. Do remember, when you sign a joint return you are liable for the entire tax amount individually and jointly unless you file for innocent spouse relief.
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