The probability of recovering something is related to the legal environment, the determination of wanting to recover and other specific of the situation. If this company does any business out of Bulgaria there may be various levers to push there. The situation may also change going forward so at least documenting the situation and the efforts may end up being able to recover something in the future. Given that Bulgaria is in the EU it seems at least worthwhile to try. Sometimes the legal environment may actually limit your ability to recovery debt (e.g. bankruptcy) so having no rule-of-law can be an advantage.

At any rate, future employability prospects should not be a factor in going after unpaid wages.

Even if Bulgaria sucks, it's a member of the EU. Workers have a right to get paid here. I don't see why prospective employers would even know that he's suing a former employer. The only real obstacle is probably that the lawsuit itself costs money, but even then, he should definitely sue.

What use are worker protection laws if employers get away with this? Only employers on the verge of bankruptcy should get away with this. If a healthy company doesn't pay its employees, it wouldn't surprise me if that's criminal. Try getting the police involved.

Note: I'm not a lawyer.