My DIL started having sex with another guy. She moved out, abandoned their kids for 2 years...and when my son finally filed for divorce, her behavior could not be mentioned in court.
Nor could he mention she had claimed the kids on her taxes while living in another state and it took 2 years for the IRS to lower the hammer. She still owes on back taxes and COVID false payments...
Your son has my sympathy. I hope your DIL was denied custody.