Sorry to hear that you are facing a sticky situation with your very first batch of tenants! This is exactly why so many landlords end up turning to an agency to manage properties - these awkward problems take up so much time and money that your DIY saving just disappears.
But it is possible to untangle this, and that's exactly why this blog exists - to be a free resource to the many entrepreneurial landlords out there going it alone. And it sounds like you have already used the advice and information to educate yourself on tenants' rights and responsibilities.
You are absolutely right, and sadly so is your rogue tenant! Much as I feel sorry for the folks he's left in the lurch, when multiple tenants are named on a tenancy agreement, they are considered jointly and severally liable for the obligations of the tenancy - including rent payments.
This comes up more often during disputes over who has the responsibility to pay for or make a repair if damage is caused to the property by one tenant - or perhaps a friend/enemy of one tenant. And the answer is that even if one tenant caused the damage (or invited the cause of the damage to a party...) then all of them bear joint responsibility.
Most of us are decent humans and if we broke something in a house we rented with our friends, we'd automatically assume the responsibility for putting that right ourselves.
However, if I've learned anything in 20+ years of sales and letting in Belfast, it's that living together will test even the strongest friendships - and we don't all act like decent humans when those relationships break down.
And that's why we have tenancy agreements!
Good luck with this one - give me a shout at the office if we can be of any further assistance to you with it.
If you have a question or a problem you think I can help with, drop me a line to firstname.lastname@example.org