Often we’re faced with situations in life when those who we care about make choices we don’t approve of [1]. Obviously if those decisions put them in harms way then we have to come forward and say something, but otherwise, our differing opinions on their lives can be taken as attacks or judgments. Whether we like it or not, sometimes it’s best to remain supportive [2] even when we don’t feel like it. To see my tips on walking this tricky line, .

• Remember that being supportive of someone doesn’t mean you have to support a particular choice they’ve made. You can offer love and respect to someone without approving of every aspect of her life.
• Instead of consistently biting your tongue, go ahead and speak up. There’s nothing wrong with telling a friend that you don’t feel comfortable discussing a particular topic with her. And she has the right to know why you might be keeping mum on a specific topic.
• Most of the time people are going to do what they want regardless of the advice they receive to the contrary, but by keeping an open dialog that communicates respect (as opposed to approval) you're more likely to not be cut out of your loved one's decision-making process.
• If a particular choice does blow up in her face as you suspect it might, try to avoid pulling the “I told you so” card. I’d assume they're already well aware, but by shoving it in their face, it’ll only lessen the chances that she’ll confide in you in the future.
• If you do feel like you must say something, propose your opinion in a way that reflects a personal anecdote. Use your own experiences to warn her, instead of just expounding your opinion.

While I never condone standing by and watching dangerous behavior take place, I do think that some people really do just have to learn things on their own, so in the meantime, it's best to practice patience over frustration.

Source [3]