They should write back advising that they do not accept the validity of the claim, and they are seeking legal advice and will be delighted to attend court to contest the issue there.
Most of these companies are chancers, relying on the intimidation of a 'threatening' letter to make people pay up.
Those that kick back are rarely pursued because it's not worth the company's time and costs to go to court, so they back off and chase someone else.
Make sure your parents keep a copy of their letter, and send it Recorded Delivery - and see what happens.
If the company write back again, re-post and we'll look at the best way forward.
Most often though, if you make it clear you will not be intimidated, these bullies do what all bullies do, and mooch off to find a softer target.