What is the ruling on dealing with a Muslim who uttered disbelief knowingly, or issued a fatwa without knowledge, such as saying that music is only reprehensible, while angry?

One should be wary of frequently associating and intermingling with such individuals, fearing their influence. Abandoning them revolves around benefit, whether present or absent. If the benefit lies in abandoning them, then that is better. If it lies in not abandoning them, then that is better. Ibn Taymiyyah considered the benefit in this matter.