Seeing best friend’s boyfriend is being unfaithful to your friend.
In Buddhism, we call that “Jyain” (literally “wicked desire”). “Jyain”, having desires on the wicked side. The word itself is already a warning.

In sutra “Madhyama Agama”, it goes “Jyain brings, sooner or later, an influence in the future as cause and effect.” That is to say, intent and actions of infidelity bring about vicious effects not only in your current life, but also in your afterlife.

To married couples, Buddha also mentioned, “enormous destruction of the whole family comes when one approach prostitutes or wives of others for feeling unsatisfied with his wife” (Sutta Nipata 106).

Why can’t we do something bad?

It is a quote from Saint Shinran, the founder of the school named Buddhist Jodo Shinshu (literally “the True Essence of the Pure Land Teaching).
“You cannot claim yourself good just because you have not killed anyone until now. You might have killed hundreds or thousands if the situation came to you. It is just a matter of destiny.” (Taninsho)

Be prepared that things all come back to you if you opt to sin

Deed that you know you should not conduct, but you are defeated by its superficially sweet temptation. Everything is destiny, good or bad, while to sin means you opt to go that direction when the situation comes to you.
So, what if you know that is something bad? You know that something you should not do. Still, you are so hooked that you cannot help and, eventually, you did that.

Down to earth, being incapable to stop something you recognize bad is the nature of human. We cannot help. Yet, we are obliged to what we have chosen.

We cannot cross the street when the signal is red.
We cannot drive when we touched alcohol.
We cannot take away stuff in shops without paying.

We all know those. However, if you have chosen to have a go even you know that should not be, you have to responsibility to bear the result.

You put your own life at risk when you cross the street with the red signal.
You should be ready to kill someone when you drive after drinking alcohol.
You should be ready to live behind bars when you commit crime.

That is it. Being unfaithful to others is the same.

It is not a sin to fall in love with someone, neither is it something judged or commented by others. Nevertheless, once you opted to be unfaithful, you are heading to that you-know-what result. Go to hell, suffer even in your afterlife, destruction is ahead.

Considering the results of being unfaithful, you listen to your heart, confront it and make up your own answer.