If you were attracted to your SO’s friend, and you were unable to control your desires and thus were willing to do the “dishonorable” thing, as @Kardamom calls it, then most likely, you wouldn’t tell your SO. Rather, you would test the waters by seeing if your SO’s friend would react positively.
If they did respond positively, you would then, slowly, take one step after another until you were having a clandestine affair. Sooner or later, you will get caught, and most likely your SO will want to break it off with you, and will also cut it off with his friend. It is also possible that your SO will stick it out, and get you to cut it off with the friend. It is quite unlikely that they will want to share you, or that you will want to keep both of them, but it happens on occasion.
The thing is, if you are going to go after the friend, you need to be pretty damn sure they are also interested. If they are not, then they might report your advances to your SO, and then you’d probably lose both. At the very least, there would be some unpleasantness.
All in all, it’s not likely to have a good outcome if you express your interest in the friend. However, love or infatuation is not always controllable and this kind of thing happens regularly. If you must do it, at least go into it with your eyes open, aware of the potential consequences and of how you will probably hurt other people’s feelings.
For what it’s worth, there are a significant number of people who do this. So I guess it’s a fairly common form of human behavior. People may believe in honor for others, but when it comes to themselves, they often behave more selfishly. Depending on what study you believe, somewhere between 5% and 60% of people cheat. That’s a wide range due to the fact that it is hard to get people to tell the truth about this issue.
Women act more moral in public about this. But unless a few women are doing all the cheating with a lot of men, then there is a lot more of this going on with women than one might think based on what they say. There are many women who are quite unhappy with their relationships. I would not at all be surprised if the amount of “dishonorable” behavior going on is a lot greater than it would be based on what people say in public.