The adverb is "friendlily"

 [https://www.yourdictionary.com/friendlily](https://www.yourdictionary.com/friendlily) 

 [https://www.thefreedictionary.com/friendlily](https://www.thefreedictionary.com/friendlily) 

"-ly" is most commonly used to end an adverb, as you point out.  But there are many other adjectives that also end in -ly, such as: cowardly, ghastly, beastly, ghostly, sparkly, bodily, giggly, bristly, godly, sprightly, bubbly, etc.  

They also form their adverb in a similar, mildly awkward way ("ghastlily").    [https://www.merriam-webster.com/dictionary/ghastlily](https://www.merriam-webster.com/dictionary/ghastlily) 

Interesting post!