Saying that dishonesty is necessary to keep a friendship strong means that there are instances when being honest with your friend could actually damage the bonds of friendship. This usually requires lying to protect their feelings. For example, if a friend loves to wear bright, Hawaiian print shirts that you think are gaudy and over-the-top, it may not serve the friendship to tell them your true feelings. This is an example of lying by omission or, not sharing what you believe to be the truth. If your friend asked what you thought of the shirts and you stated that you liked them, this would be a direct lie; a statement that is the opposite of what you believe to be true. Their common thread is that both require suppression of the truth, be it ones own internal beliefs or omissions and misstatements of fact.
Usually, dishonesty is not necessary to maintain strong friendships. In fact, it is usually our closest friends who we go to for the truth. Should a friend make an appeal for an honest assessment of their wardrobe, job, significant other, it is the duty of a friend to give them an honest but tempered opinion, to share what they believe to be true or what they know about the situation. This is the true art of maintaining friendships.
In the end, what we choose to lie about and even when we lie to our friends, depends on the nature of the friendship and even our own social skills. Being able to understand your friend's personality, their needs in the moment and the nature of the lie are key factors in making this choice. If your friend is sensitive or fragile in general or even at that instance and the lie is benign, you may choose to lie to protect their feelings. If the lie serves to betray the friend, it would always be better to tell the truth regardless of their emotional state. Being able to rely on someone to know the difference between emotional support and betrayal is what separates 'friend' from 'other'.