I am in a long-distance relationship. My boyfriend works in another province. He sends his best friend to check up on me now and then. I have since developed a crush on his friend. If I tell him how I feel, how can I be sure that he will keep the secret?
Cheating and being unfaithful can make you lose the one person who genuinely loves you.
Like you said, they have been friends before you and I believe the guy will choose the friendship over you. Think carefully about your action as it may cause trouble for yourself and the two friends.
As much as you expect your partner to be loyal, do the same.
Having a crush is normal but it is a choice to act on it that might turn it into bad behaviour. This friend might not even be attracted to you and might use you and dump you. Make a wise choice and stay committed to your partner. Good luck!