Up to 85% of SSDI benefits may be taxable depending on your overall income. See here,
https://www.irs.gov/newsroom/irs-reminds-taxpayers-their-social-security-benefits-may-be-taxable
I would fill out a new W-4 using the instructions and tables on all 4 pages, and treat your spouse's SSDI as if it was taxable wages at the 85% rate. (For example, if the benefit is $20K per year, prepare the W-4 as if the income was a wage job at $17K.)
