No. You don't write it and you don't say it (except as a joke). 'Friendlily' is NOT an acceptable word. If you were to say or write 'friendlily' in an English exam it would be marked as incorrect. We do not add 'ly' to adjectives which already have an 'ly' at the end. Instead, we say 'in a friendly way', 'in a friendly fashion' or 'in a friendly manner'.

When an adjective already ends in -ly (e.g. cowardly, elderly, fiendly, kindly, lively, lonely, lovely) we don't add -ly to make an adverb. Instead we can use a prepositional phrase with fashion, manner, or way:  - He smiled at me in a friendly way. - She waved her hands around in a lively fashion. (Advanced Grammar in Use, Martin Hewings) However, according to the Concise Oxford English Dictionary we can write friendlily.

NO

I would say you're better off saying just friendly, it sounds much better

Yes, but I can honestly say I’ve never heard anyone say it nor have I seen it written.