Will the healer mercenary heal and buff pets?

Yes, a mercenary will heal and buff a pet. It will also heal and buff you and your group members, and your group members' pets. It will also resurrect you and your group members. The Apprentice mercenrary has a 90% resurrection and Journeyman mercenary has a 96% resurrection. A merc will only resurrect or buff when everybody in the group is Out of Combat.