This loan, is it a loan from GSIS (Government Service Insurance System) or SSS (Social Security System)? Or is it a loan from the school where he's teaching? Those are the only types of loans I know that are usually deducted salary deducted. What type of loan is it? He can opt to pay it personally when he resigns. He has to contact the government agency or company (if it is) that loaned him the amount and declare that he would be paying it monthly direct from his bank account instead of getting it from the salary.

Also, I don't think they can stop him from leaving the country unless they take him to court for non-payment and he get subpoenaed by them. But before all those happen, he needs to talk to the loaning company and make some deals for continuous payment of the loan/debt. When I left the Philippines, I also had a loan from a bank that I continued paying monthly for a year even when I arrived here. So easy to do it thru online banking.