जो मेहमान यहाँ है, वह मेरा दोस्त है।

This is a very common Hindi relative-correlative structure.

• जो = who / which / that
• वह = that one / he / she / it

So जो मेहमान यहाँ है, वह मेरा दोस्त है literally works like:

• The guest who is here, that one is my friend
• or more naturally in English: The guest who is here is my friend

Hindi often uses this two-part pattern where English would usually use just one relative clause.

That is just how this Hindi pattern normally works. With जो, Hindi often pairs a matching word like वह in the main clause.

So:

• जो ... वह ... = who/whatever ... that/he/she ...

In English, repeating the pronoun often sounds unnecessary, but in Hindi it is natural and standard.

In some contexts, especially in conversation, Hindi may avoid the full pair or use a different structure, but in a sentence like this, जो ... वह ... is perfectly normal.

Because there are really two clauses here:

1. जो मेहमान यहाँ है
   = the guest who is here

2. वह मेरा दोस्त है
   = that person is my friend

Each clause needs its own verb:

• first है says the guest is here
• second है says he is my friend

So the two है forms are doing different jobs.

Yes. जो is a very flexible relative pronoun.

Depending on context, it can correspond to:

• who
• which
• that

In this sentence, since मेहमान refers to a person, English will usually translate it as who or that:

• The guest who is here...
• The guest that is here...

Yes, that is also natural.

Both are possible:

• जो मेहमान यहाँ है, वह मेरा दोस्त है
• यहाँ जो मेहमान है, वह मेरा दोस्त है

The difference is mostly about focus and style:

• जो मेहमान यहाँ है... starts with the guest who...
• यहाँ जो मेहमान है... puts a little more attention on here

Both are grammatical.

Because मेरा / मेरी / मेरे agrees with the noun being possessed, not with the possessor.

Here, the possessed noun is दोस्त.

• मेरा दोस्त = my male friend or grammatically masculine friend
• मेरी दोस्त = my female friend

So if the speaker means a female friend, the sentence would be:

• जो मेहमान यहाँ है, वह मेरी दोस्त है।

In this basic use, जो does not change for masculine/feminine or singular/plural.

So you can use जो with many kinds of nouns:

• जो आदमी...
• जो औरत...
• जो बच्चे...

What changes instead is usually the rest of the sentence, depending on number, gender, case, and the verb.

So जो itself is very stable in this kind of construction.

In careful reading, it is written वह.

In everyday speech, it is very often pronounced more like वो.

So learners should know both:

• वह = standard written form
• वो = very common spoken pronunciation

The meaning is the same here.

In this exact structure, वह is normally expected.

So the full standard sentence is:

• जो मेहमान यहाँ है, वह मेरा दोस्त है।

If you remove वह, the sentence may sound incomplete or less natural in standard Hindi.

Hindi does sometimes omit pronouns in other types of sentences, but with the classic जो ... वह ... pattern, keeping वह is the safest choice.

यहाँ means here, and it goes with the first clause:

• जो मेहमान यहाँ है = the guest who is here

So यहाँ is telling you the location of the guest.

It does not mean my friend is here.
That idea would be a different sentence, such as:

• मेरा दोस्त यहाँ है। = My friend is here.

In your sentence, here belongs to the description of the guest.

The comma is mainly there to show a pause between the two parts:

• जो मेहमान यहाँ है
• वह मेरा दोस्त है

In writing, the comma can help readability. In speech, there is often a slight pause.

Hindi punctuation is sometimes less rigid than English punctuation, so you may also see this sentence written without a comma. The grammar stays the same.