The answer is easy. Get the student the instrument they see themselves playing. If they want to play acoustic guitar, start with acoustic guitar. If they want to play electric guitar, start with an electric guitar. If the student wants drums, piano, or sax – that is the instrument for them.
It is an issue of self-image. If you fulfill that self-image, chances of success will be greater because the student will spend more time. If the student spends more time, they will get better. If they get better, they will have more fun. If they have more fun, they will spend more time, they get better. It is a circle.
When a student teaches themselves, progression is usually much slower. Experienced instructors have already seen what it takes to master an instrument. They can save a student time, and the wheel spinning that happens when a student tries to teach themselves. Use all tools available - books, friends, videos. YouTube is a great resource, but there is no substitute for a good teacher.