Friendly seems to be a difficult case:
Wikipedia considers 'friendly' both an adjective and a flat adverb:
"[...] the ending -ly is also found on some words that are both adverbs and adjectives (e.g. friendly) and some words that are
only adjectives (e.g. lonely)."
Allegedly there are street signs asking car drivers to
"Drive slow/ Drive safe/ Drive friendly!"
The dictionary Webster's Third allows "friendly" as an adverb modifying an adjective (citing W.G. Harmon writing "friendly disposed
toward the British").
Then again, there are English native speakers who refuse this usage of the word - both
Though the dictionaries Webster's Third and Merriam-Webster list
friendlily as the adverb of friendly, native speakers do not seem to be very fond of this version either.
On the LEO forum, a British and an Australian English native speaker express their
Queries on how to use "friendly" as an adverb will usually result in rephrasing of the example given or the advice to use
"in a friendly way" instead.
I have yet to find out in which cases "pleasantly" can serve as a replacement.
There is probably no better summary of the native speakers' attitude towards "friendlily" than the following dialogue:
American: "Personally, I also would not use 'friendlily'."
Australian: "That would be using language sillily."