Worst case scenario is he is convicted of OWI and then has his deferred judgment revoked for the domestic violence charge, meaning at least the mandatory minimum two days in jail for each offense would be served, perhaps consecutive to each other for a total of 4 days. There would be minimum fines imposed of $315 for the domestic assault and $1,250 for the OWI (assuming a first offense) plus a 35 percent surcharge on each, $10 DARE surcharge on the OWI (I don't know if there is also a $125 Law Enforcement Initiative Surcharge on the domestic). He would lose his licese for at least 180 days, assuming again it's a first offense OWI and no other prior alcohol related revocations on his record within the last 12 years.
Best case scenario, there's a problem with the chemical test results and the OWI is dismissed or reduced to another offense that will not cause problems with probation. Perhaps the probation could be continued rather than revoked and the person is simply held in contempt of a court order. That might be punishable by a few days in jail, but it's better than having the deferred revoked and the conviction being made a part of the person's adult criminal history.
He should report the traffic stop and/or arrest to his probation officer immediately. If the law enforcement officer suspected OWI-marijuana and requested a urine sample, then there's a chance he wasn't first placed under arrest because that starts a 45-day speedy trial clock that could be delayed by the time it takes to test urine for controlled substances. However, I would still recommend reporting the stop if that's one of the conditions of probation.
He should speak to a specialized attorney who practices primarily in OWI defense in central Iowa, preferably one who is also a member of the National College for DUI Defense. I would suggest either Bobby Rehkemper or Matt Lindholm in Des Moines.