Most adjectives of two or more syllables use "more" for their comparative form, but one big group of exceptions is two syllable words that end in -y. You change the -y to -i and then add -er: friendlier luckier easier prettier busier

In school, they told us that when there are more than 2 syllables, then use more xxly. Otherwise, xxlier. But spoken reality can differ :)

The truth is, it depends on the context in which you are using the term. You would use ' more friendly ' when comparing two people or things. You use ' friendlier ' when making a statement directly to or about someone.

both are correct, adjective vs. adverb. He is friendlier than Paul. He acts more friendly than Paul.

They have the exact same meaning, so use the one that you enjoy saying the most. What other people say is of no importance.