You have to look to protect your own financial interests and debt defaulters at the same address will affect you. You also have a problem with them essentially taking you for a ride. You can't unsee the letter and assuming that you did accidentally see its contents, you are entitled to ask what is going on and set the rules for your further support.
Was the letter left deliberately for you to 'accidentally' see it? However this happened, the issue will have to be addressed and it's likely to be a difficult conversation. It's unlikely that they're spending on socialising at the moment, so it's a worry where they money is going.
Well I’d be seriously narked off, they are supposed to be taking advantage of nominal rent and saving a deposit yet instead accruing debts. I’d ask them to explain and also advise rent is increasing....I’d select a figure that they should be saving in addition to any nominal fee you are charging... open an ISA and put the savings amount away for them each month..they can have it when they move out.Encourage them to each write out a budget and plan to pay off any debts as part of their budget
Sit down and talk to them both. Put them onto agencies that can help but do not get entangled in there financial problems. Remember to listen, do not raise your voice that wont help. There are so many tools on Martins website that can help. Do not mind your own business this is your son. I have heard of a couple who wished there son would have asked them for help, sadly it did not end well.I wish I would have had better advice and guidance from my parents (I am not blaming you or them) but money management needs to be learnt young. When it was explained to me at 40 years old that every month the interest was calculated on the whole amount, I saw stars. I corrected that immediately.Help them be better at money with money and explain all the credit pit falls.
If you plan to get a place via 'clearing'
Lower your band & save £1,000s
Past-best-before health food etc