Is it obligatory to inform a friend that it is impermissible to follow the fatwa of a sheikh who ruled that a couple may reconcile after divorcing more than three times, due to the suspicion that the sheikh is among the people of innovation, and that he should seek out trustworthy sheikhs, or should one remain silent?

If the matter is merely a suspicion not based on evidence, then one should not inform the friend, nor is it permissible to accuse the Sheikh of innovation (bid'ah), as the default assumption for Muslims is blamelessness. However, if it is confirmed that the man is an innovator, then the questioner must warn his friend about him, fearing for him from temptation (fitnah) or from adopting false statements, as Al-Nawawi mentioned in his discussion of the permissible excuses for backbiting. The friend should be advised to seek out and ask scholars whose knowledge and religiosity are trustworthy, as it is a condition that the questioner presumes the mufti's knowledge and integrity.