In life we will be faced with many challenges and decisions that need to be made. We will also make plenty of mistakes that we will learn from. . You may have a friend or best friend that feels comfortable enough to come to you during their times of need and ask you for advice. There will be times when your friends go against your advice, make decisions that you may find to be poor, or choose the option that is not best for them. Although it may be hard when you see your friend continuously putting themselves in not so ideal situations but you need to remember that it is their life and their decisions to make.
Be sure not to judge your friend. No matter how silly you may find their decisions to be, yo have to remember that you are not in their shoes. From the outside looking in, the right choice may look obvious---but the thing is, you aren't in their shoes. It may be hard at times but be sure not to look down on them or guilt trip them. Be honest with your friend, tell them how you feel about their decisions when they ask. Tough love and being judgmental are two different things.
By being supportive, I mean give them advice, tell them the truth, and help them as much as you can. You don't have to support their bad decisions, just them as people. Be a listening ear and a shoulder to cry on when things go badly or when they reap the consequences of their decisions.
You also need to accept that it is their life, their mistakes, and their lessons. You cannot control what people do with their lives or the paths they choose to take. No matter how much you want to hold your friend's hand through life, understand that you cannot.
It will be hard at times to watch someone you care about make poor choices, but it is all apart of life you cannot always prevent someone from making mistakes. You cannot save people or fight their battles for them.
On the other hand, if your friend is making decisions that are endangering themselves or their well being, you may have to seek outside or professional help for them as it may be out of your hands.