You can always like some one else even if you are in a relationship and there is nothing bad about it. But there is a very fine line between liking and loving someone . At the end of the day its you who has to draw this line as everything else depends on it. If your best friend's girl is attractive you may happen to like her. You may even fantasize to have an love affair with your best friend's girl.
Both you and your best friend's girl might be enticed to fall for each other, but most of the times it isn't really love. It is lust that stirs your mind to steal the girlfriend of someone else .
Guilt Feeling- Well if you find that you feelings for your best friend's girl is serious then you should definitely break up with your present partner and start a love affair with the girl you love. But you will be haunted by a feeling of guilt which is hard to overcome. You will also be responsible for a bad relationship between you and your best friend. This feeling can even be scary at times.
Cut Off From Friend Group- As you would steal your best friend's girl you would be looked down upon in your friend circle. Friends sometimes even boycott such people who betray their friend's trust. So its better for you to make a new circle of friends and go ahead in your lives. Forget who your friends were and move on with your new pursuits.
Break up Again- What you feel for your new partner is merely infatuation. It is primarily your desire to have a love affair with the person you are so attracted to that has given rise to this relationship. Once this lust is over, it will also be time for your love affair to be over. You will miss your best friend once you realize that you have given birth to many bad relationships. But there will be little or no time left for rectifications then.
Its better not to kindle any desire to have an affair with your best friend's girl as it completely changes everything around you. You suddenly find a new set of friends and a new atmosphere all around. All of these will come up so suddenly that it might become difficult for you to adjust. If you have encountered any such situation feel free to share with us.