Appropriate things to say to someone who is retiring include wishing him a happy retirement, saying he will be missed and expressing that he will be thought of often. Retirement is a time to offer good wishes and reminisce about positive shared memories.
If possible, be specific about what will be missed about the person and what happy memories have been shared so that comments come across as genuine. For instance, consider recalling a fun restaurant that was visited together when traveling for work. Mention some contributions the retiring person has made to the workplace.
In some cases, it may be appropriate to purchase a gift for a retiring coworker, either as part of a group or individually. In generating ideas for gifts, think about the person's hobbies or plans for life after retirement. It may be appropriate to check with the individual's spouse for ideas.
Retirement is not always a happy time, and it's important to be sensitive to the feelings of someone who may be apprehensive or who may not be retiring voluntarily. Some people worry that they will no longer feel useful. In such cases, it is appropriate to express gratitude at having had the opportunity to work with the individual. Stay away from congratulatory statements.