In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Rabee [may Allaah preserve him] said: It is not correct to apply this principle unrestrictedly because it maybe that he is unaware of the Bidah committed by that person, therefore he refrains from declaring him a Mubtadi [an innovator] due to [ورع – i.e. to take a precaution in order to avoid something doubtful out of fear of falling into something forbidden]. Why should you declare him an innovator? [i.e. why should you declare him an innovator when it is the case that he is unaware of the bidah of that Mubtadi or refrains from making Tabdee out of ورع ]. But if he knows the innovator [i.e. knows that such a person is upon bidah due to which the evidence was established against him], loves him and allies with him, then he is a Mubtadi. This is the distinction in this affair. He knows that that person is a Mubtadi but he aids him and wages war against Ahlus Sunnah Wal Jamaa’ah, then he is a Mubtadi without a doubt. As for a person who does not know that such and such person is a Mubtadi, he is not to be declared an innovator.
You [i.e. the scholar, or student of knowledge who is qualified to make Tabdee] – the one who studied the affair of [that person] and you know that he allies with a Mubtadi, places him in a favourable position, wages war against Ahlus Sunnah for the sake of that Mubtadi and for the sake of this falsehood, then this person is a Mubtadi, misguided. As for the person who does not know that indeed such and such person is a Mubtadi, then advise him and clarify for him [i.e. with clear unambiguous proofs until he understands] that such person is a Mubtadi. So, unless he ceases [i.e. refrains from defending this Mubtadi after understanding the evidences], then put him [in the same place] with his companion – the Mubtadi.
[Source: An Excerpt from ‘Awnul Baari 2/891’. slightly paraphrased]