1. What's your thought about a player telling other player how to play their character?
As long as it's just player A
advising player B, and as long as player B doesn't object to the advice, it's not a problem. Anything more than that, and it's a problem that should be dealt with.
2. As a DM, how would you resolve this issue?
Assuming it's a problem (see above), then speak to player A. Explain to him that he's causing a problem at the game, and ask him to stop. If it continues, tell him to stop. And if it
still continues, uninvite him from the game.
3. Is having a character with a boring background or playing him boring is a legitimate to tell him how to play his character?
If the player in question is happy having a "boring" background then that's his prerogative. There are plenty of characters in fiction who have a boring background prior to starting their adventures (eg D'Argtanian, who was just a kid from a farm, without even the "son of Darth Vader" angle at play). What matters is what happens in the story at the table.
Now, if the player is
playing in a boring manner, that may be a problem. Everyone at the table should be contributing to the fun of the group... or at least not detracting from it. And so
the DM should speak to that player about it, and if the styles can't be resolved then the player should be removed. But it's not the job of
another player to take action here.