Ok, first a company does not garnish, a company gets a court order called a judgement, and your boyfriend would have reicieved dozens of late notices, warnings to pay. He would have been served papers to appear in court for a hearning.
After they get a judgement then they can garnish any pay or attach any bank accounts.
And no after they get a judgement, it would be pretty silly for them to warn you they were gong to get your bank account, no one would leave their money in it would they. So no after the judgement no other warningis normally needed. In TN and a few other states prior notice to a paycheck garnishment has to be given on an approved state form. ( but many places don't do it, anyway)
Now they can not get his disabillity benefits, but now that his bank account is frozen, he is going to have to prove by motion with the court what percent of the money in his bank account was from his disability and how much was from other sourses.
He will need an attorney for this ( and he needed one when he was served with notice of the judgement hearing)