Praise be to Allah.
We have previously spoken about the Habashi sect and explained the beliefs that they have that are contrary to the beliefs of Ahl al-Sunnah wa’l-Jamaa’ah, in the answer to question number 8571.
A number of scholars have warned against their misguidance and deviation and some books have been written about that which it would be good for you to refer to.
With regard to advising this man who belongs to al-Ahbaash, that should be done by a capable seeker of knowledge (taalib ‘ilm) who has studied their misguidance and is able to respond to it properly, and who is also able to explain the beliefs of Ahl al-Sunnah wa’l-Jamaa’ah properly. If that is not possible, then you can give him a book which refutes them without getting involved in debates or arguments if you is not qualified to do that, especially in front of common folk who may be influenced by these ideas.
The advice should be given with wisdom and beautiful preaching. Some people join this sect out of ignorance of its beliefs and because they think well of its members, and giving him advice in a harsh manner may prompt him to adhere more firmly to it and defend it.
Advice should also be given to the wife in an appropriate manner to beware of the misguidance of this group.
And Allah knows best.