I was watching a religious show on TV, there was this Mufti who said that we should not offer salah behind a Deobandi since they say bad things about Rasool Allah. How should we reply to such people?
Sometimes best response to ignorance is not to engage in discussion at all. In describing the qualities of Ibadur Rahman (true servants of Rahman) Allah Subhanahu Wa Ta'ala mentions that "When ignorants address them, they (true servants of Rahman) say Salam." By saying Salam, you are simply moving on and not engaging at all. The comments of Mufti that you are referring to appears to be based on ignorance or misinformation because the reality is not as described by him. Therefore, you do not need to respond. If someone sincerely seeks an answer or seeks a solution then can have their specific concerns addressed by a scholar who will be happy to answer those questions.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America