What a helpful friend he is ? What is the subject and predicate in this sentence

Although you have placed a question mark after the example sentence, it is actually a declarative sentence. The sentence makes a statement.

A predicate is the verb plus all the words that follow that are related to that verb (a sentence can have more than one verb/predicate).

In the example sentence, the word "what" is functioning as an adverb, modifying the verb "is" as "to such extent".

The adverb introduces the adverbial phrase "what a helpful friend".

A phrase is a group of words based on a noun (friend) that functions as a unit in a sentence, in this case, as a predicate noun.

A predicate noun (also called a predicate nominative) is a noun, a phrase, or a pronoun that follows a linking verb and restates the subject.

A linking verb is a verb that acts as an equal sign, the subject of the sentence is or becomes the object. In this sentence, he = friend.

A predicate noun is a type of subject complement. The other type of subject complement is a predicate adjective, which is an adjective that follows a linking verb and restates the subject. Example: He is funny. (he = funny).

Subject: "he" Predicate: "is"