General rule of thumb is, if the 3rd party program in any way sends commands to the game or allows the manipulation of something that normally you can't, it's illegal to use in game.
Parsers normally only read the information that's available and process it into a format that makes it easy to compare. If that's all the program would do then you're fine.

If it starts doing more things though like say, automatically performing actions for you to Maximize your DPS or Mitigation, then it's not ok to use.