I’m preparing taxes for a relative and their employer changed them to tax exempt without their permission in March 2024, resulting in a large tax bill. Any advice?

They have to pay the taxes owed, that is a given.  If they cannot pay the full amount, then they should pay as much as possible now, on or before April 15.  Then go to this IRS website to set up a payment plan - https://www.irs.gov/payments/online-payment-agreement-application

And make sure that they are currently having federal taxes withheld from their wages on each pay stub.