Having dealt with something similar in another large local company I can only suggest that you find out what your companies policy is on ill health absence/retirement before the meeting, most good organisations post these on their intranet etc. Also if your colleague is in the company pension fund they (via the trustees) will probably also be involved at some point so again they may have a published policy on the matter. Such retirement will not be decided on in one meeting and medical professionals will need to be involved.
As suggested above the ACAS site is a good source and remember do not agree to anything without taking expert advice, treat it as an "information" meeting.