A "friend" by definition is,
a person attached to another by feelings of affection or personal regard.
a person who gives assistance; patron; supporter: friends of the Boston Symphony.
a person who is on good terms with another; a person who is not hostile
If a person is trying to undermine you by "keeping the client from choosing you over them by lying about your skills and talents and consistently gossip about you," then by definition they are NOT a friend, so my answer is a NO. Having negative people in your life who bring you down or in this instance try to take you down is not healthy for you. You should always surround yourself with people who lift you up and inspire you. It is time to make a break with this person! Don't let their "sucking up" interfere with what needs to be done, it is obvious they are two faced. You need to be clear, concise and expedient!