Firstly, most insurers only recognise 5 or 6 years of no claims - some may recognise a bit more, but not 28!
Secondly, the bonus is only protected if the policyholder has requested it, and paid an extra premium (normally between 5 - 10 %)
Thirdly, if the bonus is protected and a claim is made against the policy, then only a 'life' is lost - but the discount off the premium remains the same - as long as they renew with the same insurer. Some companies will transfer the bonus, but the choice is limited on the open market.
Hope this makes sense - it does to me, but I've worked in insurance for about 18 years!