Muy amigos

The construction muy amigos appears curious to English speakers, since it looks like *very friends, an ungrammatical utterance in English. Nevertheless, this is possible in Spanish.

• Somos muy amigos él y yo. We are great friends. Note that this doesn't mean they are friendly in general, which would be amigables.
• Soy muy amigo de tu hermano. I'm a good friend of your brother's.
• ¿Eres amigo de Diego? — Muy amigo. Are are a friend of Diego's? — Yes, actually I'm a very good friend of his.

To restore grammatical order to the universe, we can either consider that an adverb may modify a noun; or that we have a kind of fused adjective-noun; or that this is elliptical for Somos muy buenos amigos.

Here's an example with a different noun: Es muy fan de los 49ers. He's a big fan of the 49ers.

Tan amigo

The intensifier tan can work similarly: Y como Rocío es tan amiga de Sofía ... And since Rocio is such a good friend of Sofia's ... (telenovela dialogue)

Es muy tu

A similar use of muy crops up in expressions emphasizing independence, such as es muy tu vida.