I'm a bit confused by this: "hard to find mortgage interest monthly since every month the rent expense should be reimbursed. How to find it?"

Nothing should be reimbursed. That's the point. A landlord/owner has costs. They might or might not also have income.

The company has rent expense. It isn't reimbursed. If he is paying rent out of pocket (because the company doesn't have the money), then something else is wrong here. Perhaps he needs to put money into the corporation to bridge lean times (loan from shareholder or additional paid in capital)?

There should be some respect paid to the separation of entities. The corporation isn't his own piggy bank from taking money out, and it isn't his spouse or sibling for his payment of business costs. There should be a bit more formality in the movement of funds between two independent entities: self and corporation. Otherwise, why did he incorporate?