I'm in a relationship with my partners closest friend and have been for nearly a year. I think the only person who can really tell you if you should act on the attraction or not is you. You'll probably want to consider things like how strongly you are attracted to the friend. If you do decide to persue this I would advise telling your partner that you have an attraction to his friend and may be interested in acting on it but would like to hear his (as in your partners) views before acting. I would imagine that starting with an open relationship could help.
Hope this helped!