दोस्त आपस में बातें करते हैं।

Because दोस्त is one of those Hindi nouns that often looks the same in both singular and plural, especially in direct case.

• एक दोस्त = one friend
• दोस्त = friends

Hindi does not always add a visible plural ending the way English usually does with -s. You often understand the number from the verb:

• दोस्त ... करता है = a friend ... does
• दोस्त ... करते हैं = friends ... do

So in this sentence, करते हैं tells you that दोस्त is plural.

आपस में means something like among themselves, with one another, or with each other.

So:

• दोस्त बातें करते हैं = Friends talk
• दोस्त आपस में बातें करते हैं = Friends talk among themselves / Friends talk to each other

It adds the idea of mutual interaction. The friends are not just speaking in general; they are talking with one another.

In Hindi, बातें करना is a very common expression meaning to talk, to chat, or to have conversations.

• बात = a matter, a thing said, a conversation point
• बातें = talks, conversation(s), things said

Even though English usually just says talk, Hindi often uses the plural बातें in this expression. So बातें करना is idiomatic Hindi, not something you should translate word-for-word too strictly.

Hindi often uses noun + करना to make common expressions.

Here:

• बातें करना = to talk / to chat

This is more natural for having a conversation than simply using बोलना.

Compare:

• बोलना = to speak, to say
• बातें करना = to talk, chat, converse

So दोस्त आपस में बातें करते हैं sounds very natural for friends talk to each other.

Because the verb here agrees with the subject दोस्त, not with बातें.

In this sentence, दोस्त is understood as masculine plural or a mixed-gender plural group, so the verb is:

• करते हैं = masculine plural / mixed plural

If the subject were definitely an all-female group, you could say:

• सहेलियाँ आपस in बातें करती हैं।

So even though बातें is feminine plural, that does not control the form of करना here.

हैं is the present-tense form of to be used here with a plural subject.

In करते हैं, the full verb phrase marks habitual or general present meaning:

• करते हैं = do / usually do / habitually do

So the sentence means something like:

• Friends talk to each other.
• Friends usually talk among themselves.

If the subject were singular masculine, it would be:

• दोस्त ... करता है

If plural, it becomes:

• दोस्त ... करते हैं

It can include women too. In Hindi, the masculine plural form is commonly used for:

• a group of men
• a mixed group of men and women
• a group whose gender is unspecified

So दोस्त आपस में बातें करते हैं can mean friends talk to each other in a general sense.

If you specifically mean an all-female group, Hindi would normally use a feminine subject and feminine verb form, for example:

• सहेलियाँ आपस में बातें करती हैं।

Yes, it can be omitted:

• दोस्त बातें करते हैं। = Friends talk / Friends chat.
• दोस्त आपस में बातें करते हैं। = Friends talk to each other / among themselves.

Without आपस में, the sentence is still correct, but it is a little less explicit about the mutual with each other idea.

By itself, करते हैं usually gives a habitual or general meaning:

• Friends talk to each other.
• Friends tend to chat among themselves.

If you want to clearly say it is happening right now, Hindi more naturally uses a progressive form such as:

• दोस्त आपस में बातें कर रहे हैं। = The friends are talking to each other.

So your original sentence sounds more like a general statement than a description of an action in progress.

Not really in the same form, because आपस में implies more than one person. A single friend cannot talk among themselves in this sense.

So:

• दोस्त आपस में बातें करते हैं works because there are multiple friends.

If you want singular, you would need a different structure, such as:

• एक दोस्त दूसरे दोस्त से बात करता है। = One friend talks to another friend.

So the idea of reciprocity in आपस में naturally requires a plural subject.