Thought someone might have a suggestion of an existing add-on or built in EE solution.
What the client needs is a way to trigger a specific 'reminder' email sent out. The date, time, and email template, would be set in fields on the individual members entry (using members as entries).
For example, an entry called "John" could be updated by the client to trigger a reminder email to go out in 30 days, which would only go to him.