to tell your bud that he has a huge fucken snot hanging in his nose?
If he is your "bud," you are obligated to tell him. In fact, he'll appreciate it. Imagine talking to a hot chick, going to the restroom to notice you have snot hanging from your nose, then realizing that your so-called "bud" would have seen it just before that and could have warned you... You can and should tell him and they should appreciate the fact that you're looking out for them.