Really? Your friend is paying her soon-to-be-ex-husband's child support? Wow.
Yes, he could go to jail if he doesn't pay the support. If he doesn't pay court ordered support he could be found in contempt and jailed. A lot depends on whether he has a history of not paying support. A judge may be more inclined to give him a chance if this is first time falling behind, but not if he is a repeat offender.
And yes, he is still liable for monthly support payments if he has no income. He can petition the court to modify his order if his financial circumstances changed. But if the court finds he should be working and his not working is of his own choice it may impute income.