Fiance put kisses at end of instant message to fellow colleague?

Definitely confront him. Don't go into a marriage without being completely sure that he's faithful! Sit him down for a casual conversation, and once you get speaking about this girl, set the sit-down to a more serious tone. Make sure he understands that it isn't okay to go around flirting with other girls when he's engaged--- let alone right in front of his fiance!