Mi amiga tiene el pelo moreno y muy largo.

Spanish often talks about physical features with tener + noun, not with to be as English often does. So tener el pelo, tener los ojos azules, and tener barba are all very normal patterns.

That is why Mi amiga tiene el pelo... sounds natural in Spanish.

Because the owner is already clear from mi amiga. In Spanish, with body parts and closely connected physical features, it is very common to use the definite article el / la / los / las instead of a possessive.

So tiene el pelo moreno is the normal, idiomatic way to say it.

You could say su pelo for emphasis, contrast, or clarity, but it is not necessary here.

With pelo, moreno means dark-haired or dark in color, usually brown to dark brown rather than blond.

In other contexts, moreno can also describe a person with dark features or tanned skin, but here it clearly refers to the hair because it modifies pelo.

Not exactly.

• Castaño is more specifically brown / chestnut-colored
• Moreno is broader and often means dark-haired or brunette

In everyday Spanish from Spain, moreno is very common when describing hair, especially in a general way.

Because those adjectives describe pelo, not amiga.

• amiga is feminine, but it is not the noun being modified by moreno and largo
• pelo is masculine singular
• so the adjectives must also be masculine singular: moreno, largo

Spanish adjectives agree with the noun they describe, not automatically with the person being talked about.

Because in Spanish, descriptive adjectives usually come after the noun.

So these are the normal patterns:

• pelo moreno
• casa grande
• coche rojo

English often puts adjectives before the noun, but Spanish usually puts them after it.

Because muy is used with adjectives and adverbs.

• muy largo
• muy bonito
• muy rápido

Mucho is usually used for quantity:

• mucho pelo
• mucha agua
• come mucho

Since largo is an adjective, muy is the correct word.

In moreno y muy largo, muy only modifies largo.

So the sentence means that the hair is:

• moreno
• and very long

It does not automatically mean very dark too. If you wanted both adjectives intensified, you would usually say muy moreno y muy largo.

Both can mean hair, but there is a small difference in use.

• pelo is the everyday, most common word
• cabello is a bit more formal or literary and refers specifically to head hair

Also, pelo can mean body hair or even animal fur depending on context, while cabello is normally just hair on the head.

No. Spanish often leaves out subject pronouns when the subject is already clear.

Since the sentence already begins with Mi amiga, there is no need to add ella.

You would use ella only if you wanted extra emphasis, contrast, or clarity.

Yes, that is a natural sentence too.

The difference is:

• es morena describes your friend more generally as brunette or dark-featured
• tiene el pelo moreno talks specifically about her hair

So the original sentence is a bit more explicit about what moreno refers to.