Derek Hayes works for Cardinal Emmett Fairbanks as his physician, and also his informal confessor.
When Fairbanks is under consideration for becoming the new pope, he tells Hayes about a variety of his worries and problems. The biggest problem is a drug his diocese is developing that unfortunately made its trial monkey speak English and then kill itself. The Church would not approve of this.
Hayes passes this information on to Fairbanks' rival, Cardinal Robertson, in return for a new job with him.
While Hayes certainly stabs Fairbanks in the back (metaphorically), he does seem to have a sincere interest in his health, particularly in trying to get him to stop taking stimulants.