Depends
If he can be countered with facts, then do so. What are the reasons
why his company is labelled as fraud? Gather more info on them and
present it to him.
If he is of the other type, those chaps who like to talk MLM as if
they are doing a thesis on philosophy, my advice is to let them be
and learn the lessons.
Notwithstanding, if he is found to be doing well and it seems his
company is A ok, well, i guess as a friend you have to encourage
him.