Hey Marc - from what I gather your friend is already in a relationship but is infatuated with SOMEONE ELSE - am I understanding correctly?

If so perhaps using Creating Sparks and/or Positive Relationships about his longtime partner instead of his infatuation might help him.

Usually if a persons attention and affection strays from their main relationship it is because their needs aren't being met. If he concentrates on his "real" relationship it may help his infatuation fall by the wayside a bit quicker.

I say this as a believer in the Law of Attraction and that you get what you think about it and if your friend is constantly fearing he will stray from his partner that might be exactly what occurs.