True friendship isn't dependent on your saying "yes" all the time and overlooking your friend's faults. Of course, while you're tolerant of their idiosyncrasies, you'll also want to support your friends in their personal development.
For instance, if you feel a friend drinks or smokes too much, you can tell him so. Offer some words of wisdom. Most of all set a good example.
Love can be both tough and gentle in its attempt to further the welfare of a friend. If your motives are pure, your friends will always be grateful, even if they initially seem to resist your efforts.