Yes, there's a special carve out in the T&Cs that allows this, to apply the two vouchers to one booking. But this is where both vouchers are being used as "2-4-1", so 4 people for the price of 2.

But as I said in my initial reply, they don't allow the vouchers to be used in "mixed" way, where one provides a for a free passenger, and the other provides a 50% discount.

It's just a policy decision and it does result in oddities like this, but unfortunately he has been advised correctly. If you have 2 vouchers in your account you can verify this with a dummy booking online. Even if you apply the 2nd voucher, the third passenger will be charged the full amount.