That's (someone's) problem spoken language informal
Said to express your opinion that someone should address their own issues.
A: Can you help me write the essay? B: No, that's your problem. I've already suggested some ideas to you.
A: John is asking for financial support because he had gambled all his money away. B: That's his problem. I'd lent him money several times, and he'd never paid back!
A: Mary is increasingly despondent over losing her earnings on gambling. B: That's her problem. I told her to stay away from that evil, but she didn't listen.
The origin of this phrase is not clear.
A legal matter that is very easy to solve
The house owners didn't lock the doors before going out, and that's why they got broken in. This is an open and shut case.