If you happen to fell for a friend's lover, you should consider that it is likely to lead to the destruction of the friendship if the other person finds out. Therefore, the first thing to consider is whether it is okay to sever ties with that friend.
People do not often have lifelong friends. Even with friends from school, it is common for them to gradually become distant after entering society and eventually become complete strangers.
Furthermore, love is free. If both are in love, no one has the right to stop them. Choose between love and friendship... Please be honest with your feelings.
Approach the person with good intentions naturally, and gradually appeal to your considerate side. In front of the person, it will create a better impression if you have many words of praise for your friends. The key is to make the person feel drawn towards you.