Adverbs often end in -ly. But the word friendly is not an adverb, is it? A friendly advice is incorrect, but a friendly person is correct. Is the word friendly very unusual or are there many non-adverbs ending in -ly?
This is fundamentally a historical question, and if asked on ELU would deserve a very long and interesting answer. Here I will offer only so much history as might help a learner avoid confusion.
The -ly ending on adjectives descends from an Old English suffix -lic which was very often employed to turn a noun into an adjective. Consequently there are many adjectives today which have the -ly suffix: manly, womanly, daily, kingly, cowardly, to name only a very few in addition to your friendly.
This use declined in Early Modern English, and today it is no longer ‘productive’—that is, we no longer employ the ending to create adjectives. (Today we mostly use the -ish or -like or -y suffixes, or just use the bare noun as an adjective, or create an adjective from Greek or Latin roots.)
The -ly ending on adverbs descends from a very similar OE suffix -lice. When the adjective -ly fell into disuse, the adverbial -ly had the field to itself. Well into EModE, however, it was more usual to use the unmodified adjective in an adverbial sense. In the 17th and 18th century, however, there was a strong movement towards rationalizing the written language; and at this time the -ly ending became what it is today, the standard and almost universal way of distinguishing an adverb from its corresponding adjective.
Words ending in -ly can be adjectives and adverbs, although -ly is better known as an adverb suffix.
Yes, friendly is an adjective formed from the noun friend, but you cannot convert friendly in an adverb adding another -ly: *friendlyly.
So, you have to use friendly in both cases.
Other adjectives ending in -ly could be, for example, lovely or scholarly.
(Ref. English Grammar Today - Cambridge)
-ly is the suffix used to form adjectives, and adverbs from adjectives.
In the first case, the adjectives have two meanings:
- having the quality of (brotherly)
- recurring at intervals of (hourly, quarterly)
As far as I know, friendly advice is correct: Friendly is also an adjective and, as in friendly person, it is followed by a noun. Friendly can be used as adverb, at least in American English; alternatively, friendlily is adverb too.
In British English, friendly is adjective, and noun; the adverb is friendlily.