En refers to heureux, i.e. the customers, the professionnels.

Elle en a fait des heureux

=

Elle a rendu heureux beaucoup de professionnels

I would say “en” simply refers to “des heureux” and the object is repeated for emphasis. I would add a comma for better comprehension: “Elle en a fait, des heureux”, where “des heureux” comes as a precision. It would be equivalent to drop the “en”: “Elle a fait des heureux”, which is a colloquial expression.

Another possible reading is that “en” refers to “les professionnels”: it made professionals happy. Then the expression would be “faire de quelqu’un un heureux” but it seems less colloquial to me.