With all respect, I must disagree with you, Francois, on this one.
In my opinion, 'friendlier' is the preferable comparative of 'friendly', similarly to 'pretty' - 'prettier', at least in formal written English. The forms 'more friendly' and 'more pretty' certainly do exist, but I'm not sure whether they can be regarded as correct.
Sure enough, we say 'more easily', 'more slowly' etc. but those are adverbs. The adjective 'easy' forms its comparative 'easier', and 'friendly' is an adjective, too.