Either he shows them or he tells them. Either way, it will lead to the other being done after the other one. All in all, it actually depends on your friend's decision and passion.
Let us have the premise that your friend's passion and decision is firm.
1.) It is said that "Actions speak louder than words", so this might be able to help convince his parents. Let your friend be more immersed in his interests such as joining organizations and clubs and bring home results that would make his parents think that he really is serious about his interest such as trophies, medals, certificates, and the like. After which, either the parents or your friend initiate the talk about this.
2.) Let him talk it out with his parents. This would be, if not, the best way to start convincing your friend's parents. This can be done when both parties are present, most likely during dinner where family matters are commonly discussed in families. Open up the topic by making them ready to listen. Something along the lines of "I want to tell you something important". Tell them honestly and sincerely, no faking it since it would only cause more trouble. This would usually lead to them saying no to your friend's decision. When this happens, you take on a gamble if you are ready. This is where showing them enters the scene.
3.) Do both. Talk it out first and show them. That would be ideal for everything. At least both your friend and his parents are on the same ground.
If your friend's passion is already dwindling and might not be long when it goes down the drain, then he needs some thinking about what to do. No one can decide for him unless he lets them. If he lets them decide for him, it was his decision. He will never be forced unless he lets them force him.