First of all, he has to be made redundant to be eligible for a redundancy payment.
If the person has been on sick leave until now, and hasn't had their contract terminated, then they are still employed, and their service is considered to be unbroken.
For the last three years of employment, the first 26 weeks of sick leave is counted as service (this is increased to 52 weeks if the injury/illness occurred in work) when you are working out how much service the payment is based on.
For periods that occured longer than 3 years ago, all service is fully reckonable.
That means in the above circumstances, if your uncle makes this employee redundant, he will have to pay him a redundancy payment for all of his service up until 3 years ago. The last 3 years will be disregarded.