Both friendly and friendlily are Standard as adverbs, with the second used the more frequently, but most people aren’t very comfortable with either. The problems are that friendly is too strongly an adjective (He was a friendly man) and friendlily is too cumbersome and tongue-twisting an adverb. In Formal situations we often turn to a circumlocution such as in a friendly way or in a friendly fashion, and Conversationally, especially in Casual dialectal situations, we may turn to locutions such as friendlylike, with -like underscoring the adverbial function of the word.
Similar problems arise and similar solutions serve when we need to make adverbs of other adjectives that end in -ly (e.g., lovely, lively, timely, lonely, kindly).
