Here is what I would do
1, Write to the customer and tell them you will be in touch within 7 days when you have taken advice.
2, Find a fitter with a can do attitude and powerstretcher, get him to stretch it back, while not admitting any liability, just say it is out of goodwill.
3, Ask the manufacturer for cleaning recommendations,and fitting procedures.
then..
a, If they say HWE ,or the carpet is fitted differently to manufacturers guidelines, write to customer, saying that you followed manufacturer guidelines, thankyou goodbye. or..
b,If they say do not HWE , get 2 quotes, then offer customer the lowest amount, LESS 40% (4years usage they have had out of 10 years life expectancy) LESS your cleaning cost LESS £500 deposit payable when you can collect the old shrunken carpet.
If the customer disputes or will not allow this , which is reasonable and following YOUR procedures. Then let them take you to small claims ,, which will take them months, and cost you little more.