It usually happens at the worst possible moment — bedtime, a school night, right when you've said no. Your kid looks at you and says some version of: "Well Dad lets me play as long as I want."
Here's the thing worth knowing before you react: this line gets used whether or not it's fully true. Kids aren't lying exactly — they're testing. They've figured out that inconsistency between houses is negotiable ground, and they're going to stand on it as long as it works.
What not to do
Don't argue with your kid about what happens in the other house. You don't have visibility into it, and arguing puts you in the position of contradicting them in front of them, which rarely lands well.
Don't text your co-parent mid-conflict, either. A "did you REALLY say he could stay up all night on his tablet?" message sent in the heat of the moment usually starts a fight that has nothing to do with screen time anymore.
What actually works
Say something simple and boring: "That's a good thing to bring up — I'll check with Dad, and the rule here is still the rule here." Then actually follow up with your co-parent later, calmly, not as an accusation.
The longer-term fix is getting ahead of this entirely with a written agreement both houses have already signed off on — so there's no ambiguity for a kid to test in the first place.