What would you call a person who does not understand who is his/her well wisher and who is their Ill-wisher?

A naif, pronounced na-EEF. It's been used in English since the 1500's as a word for "a naive person". According to this definition at Collins, however, its use has declined over the years, so it has the double effect of sounding slightly out of touch as well as foreign.

That said, the best word depends on your assessment of the person. A naive person can become informed, so naif signals the possibility of improvement. It can be a tactful way to convey your meaning.

There are other types of poor judgement that are not so easily changed, and plenty of words to describe such people, many of which are not tactful at all.