There are a few things u can do in this situation.
I dont care if the story that u have said is true or not. But its a problem which i would like to put my opinion on.
1) Since u mentioned his family is here, does his wife have a job in Qatar. If yes, is she in any position to help financialy. The people who know him very well and know that he is not a cheat have to step in. Example people like u and ur husband. 2 people guarenting for a guy would ensure that, due to the loan he has with the bank he wont wind up in jail(2 guys as garantors would be an exception given by the bank). There is a lot of math calculation involved in this, in regards to the Obligation criteria which is 50% as per QCB, So u need some one as a garantor who has a handsome salary and a minium Obligation or a QATARI GUY.
2) If the wife doesnt have a job she needs to search for one. Or she should take her children and leave Qatar ASAP. and find a job back in her own country. to make a contribution (financialy) to solve this matter.
3)U mentioned in ur post that they used the money to build a house back in their home country. I dont see any need for that now. Ask them to sell it off and pay a partial payment to the bank. Infact in ur post u mentioned he has paid already for 4 yrs. If so his debt to the bank would be nearly bit more than 1/4th of the loan amount he has taken. So by selling property that they have, he can pay of the loan and a handsome amount on his credit cards. Doing this he will buy time for himself. To look into making serious money, finding a Job etc.
4)For the cheques that have bounced he needs to spk to the bank and the legal Dept and find a solution since the case has already been registered.

And ofcourse make sure he gets in touch with a lawyer for legal procedings.

FYI
THE GUYS WHO FEEL THAT 300,000 Loan is hard to get. Sorry to say, but u guys were so sleepy that the world around u changed, but u were not able to notice. Before the finincial crisis (resession) happend banks were lending like crazy and this guy took loan 4 yrs back when DOHA WAS ON A MAD RUSH TO GIVE LOAN, THE GUYS WORKING IN PERSONAL LOAN DEPT MADE A COMMISION OF 65,000Qr per Month. Ask any seasoned person who was involved in this mad rush. He took a huge loan sent it to his home country. Invested in fixed deposit. The interest he got from inital deposit was more than the interest he paid here. Or he invisted in property.
But thats maths that u guys wont understand cause ur tag line is " I AM CONTENT WITH WHAT I GET" the best tag line for a looser.

In between Mariz if u need help mail me @ [email protected].

"THE BIGGEST RISK IN LIFE IS NOT TO TAKE A RISK"