If he is taking anticoagulants fish oil is counterindicated.
However, diet is still the best bet for improving his condition if it can be improved. There is a wide range of causes and levels to which it can have progressed.
You are correct that he should not be taking anything to boost his test or any fat-loss type supplement. His best bet by far is to realize that he must be vigilant with what he eats. That should at least help with the fat and take some of the load of an already stressed organ.
I cannot make any recommendations for strength because you have not given any indications of the severity of the situation. Even if you did, I would be hesitant to recommend anything unless it was a first detected case and he has no other heart issues.
The best bet is to talk to the cardiologists and make it clear that he is willing to go the extra mile (he is right?) to make sure he does not do something to harm himself. This can be relatively minor or extremely serious. Not even an expert in the field is likely able to give you a clear answer over the internet.
If he has issues with his doctor, he can consider a second opinion from another expert in the field.