It is, of course, ultimately the insured's responsibility to check that their insurance documents are correct, but in the situation you describe (if my understanding is correct) your friend gave their correct date of birth but the insurance company keyed it into the documents incorrectly.
Regardless of what their insurance company told them, this would not result in the non-payment of a claim. There is no insurance company in the UK that could rely on a mistake they made to repudiate a claim. The FSA would crucify them.
I strongly suspect your friend spoke to a 'keyboard jockey' who, more often than not, doesn't understand the product they are selling, their responsibilities under ICOBS or the legal minutiae of insurance.
Had your friend given the wrong date of birth, and the insurance company charged a lesser premium as a result, then they could argue it was misrepresentation and void the policy. However, if it could be demonstrated it was an innocent misrepresentaion, then the proportionality procedure would apply whereby a claim payment is reduced in the proportionately based on what the insurance company would have charged had they been armed with the correct information (the proportionality procedure wouldn't apply to payments that must be made to a third party).
What you have is anecdotal evidence - which of course isn't a substitute actual evidence.
It never ceases to amaze me how much insurance is misunderstood - for example the number of times I have seen on this site people wrongly advising others that the absence of an MOT and/or Tax invalidates insurance.