Everyone has to start sometime so you can't fault the person for wanting to plunge in. But, what you do know about his interior painting qualifications? How are they going to prep for the exterior work? Will they be able to replace rotten wood if they find it? Have you researched picking paint for vinyl i.e. don't use a color darker than what exists?

As long as you make sure he knows what to do and can access resources to help him, it might be worth giving him a try if his interior references are excellent.