on a joint 2018 return you don't split income her agi is the same as what was reported on the 1040 line 7
since she is on the return, she may be able to get a transcript of the 2018 return which will show the AGI
https://www.irs.gov/individuals/get-transcript
by the way he may have been breaking the law by filing without her permission. the problem would be proving that to the IRS it would be a she says he says case.
