That's good that you're going to make him wear a helmet.
The first time I rode "the shore" it was definitely an eye opener. I didn't have any armour either - I honestly had no idea what the trails were like. I was a good xc rider, who couldn't understand why anyone would want their seat lower than their bars. I grew up in Alberta, there aren't many hills there. Luckily, I had a great tour guide (thanks fergs!) who showed me the ropes. So my advice, would he to go with a larger group, or at least someone who knows what they're doing if you can. My perspective on mountain biking changed, maybe his can too?