"Ein ungefragter Rat ist ein Schlag."
Unfortunately, in english, it doesn't rhyme that nicely: "An undemanded advice is a punch." (I seriously doubt that "undemanded" is the best translation for "ungefragt", but you get the idea.)
When feel like people would benefit from advice to the extent that not giving it would be unfriendly/selfish/whatever itself, I try to follow following guidelines:
* Refraining from using "you did", "you said", "you <whatever", but instead choosing something like "I read that as", "I interpreted it this way". * Not implying what the other one meant, but instead saying something like "when I did it the way you did, I did it because of xy. Only later I learned that I was wrong, and people didn't actually mean it that way." That puts you onto the same level as the person you are talking to. Even if the other person takes what you say as accusation, you are accusing yourself of the same, so it's less bad. Plus, there is still the emergency exit for the other person to say "well, actually my reasons for stating it this way are entirely different".
I hope you get what I mean.