Beginner questions

I've been reading Tae Kim's grammar guide to learn Japanese grammar. He explains in the "descriptive subordinate clause section" that the declarative だ can't be used at the end of a descriptive subordinate clause to modify a noun. So apparently a clause like 友達だ人 is incorrect but 友達じゃない人 (roughly meaning "person who was not a friend") is. He doesn't really explain how to compose a grammatically correct affirmative-present tense-state of being-clause in the next section so I'm confused.

How do I write a sentence that roughly means "person who IS a friend" ?

Would 友達の人 have that meaning? Or do I just not use a particle? Please don't only suggest an alternative way to phase this expression. I'm mainly interested in composing a sentence with this structure (if even possible) for the sake of completeness.

Another question I have pertains to the particle に. In the clause 艶やかにマニキュア を塗っている is the に used to indicated that the na-adjective 艶やか is an adverb that modifies the verb 塗る.

Kim's guide seems to affirm this statement however he is quite ambiguous about it stating that some people don't even consider this construction to be "an adverb and verb." The guide is also really unclear about word order in a sentence. Would it be acceptable (if 艶やかに actually is an adverb) to place it after を and before 塗っている)?

Thanks.

Gramatically speaking, when you want to use a copula to modify a word you -can- say 友達である人 , but である is stuffy sounding, it's used mostly in writing, narration, and such. な as seen at the end of な adjectives is also a form of the copula, or perhaps it's better to say 'derived from the copula' since you can't use it arbitrarily and can't use it here.

In most cases of normal speech, I think you -do- simply rephrase to avoid modifying a noun with a copula form.
友達である人 is of course an unnatural phrase since it just means 友達 and is completely redundant. 友達の人 would also be redundant and isn't used. If 人 is replaced with Aさん though, then you can say 友達のAさん to mean 'My friend A'.

And yes, when な turns to に it's an adverbial construction (whether it's actually 'an adverb' or not is neither here nor there to me, it acts like one.) And yes, you can move the な形容詞＋に construction to right before the verb. Kim is often vague on order because he argues that Japanese word order is completely flexible in that way, that is, freely reordering your をーmarked phrase with your にーmarked phrase, etc. for all the case particles. (は・が・を・に・へ・で・maybe others, I don't know if he meant to include particles like より・まで・から in that argument.) Of course, if you reorder too freely you may have an odd-sounding sentence even if it is comprehensible but it's not a bad working theory.