watasi no tomodati ha byouki nanoni zangyou wo site ite, kawaisou desu.

の shows possession or connection. So 私の友達 means my friend.

Literally, it is something like:

• 私 = I / me
• 私の = my
• 私の友達 = my friend

This is one of the most basic uses of の in Japanese.

は marks the topic of the sentence. Here, 私の友達は means as for my friend...

The sentence is about that friend, and the rest of the sentence says something about them:

• 私の友達は = as for my friend
• 病気なのに残業をしていて、かわいそうです = even though they are sick, they are working overtime, and I feel sorry for them

So は is not exactly the same as an English subject marker. It introduces what the sentence is talking about.

Because のに attaches differently depending on what comes before it.

病気 is a noun in this sentence, so you need な before のに:

• noun + なのに
• な-adjective + なのに
• い-adjective + のに
• verb + のに

So:

• 病気なのに = even though someone is sick

You cannot say 病気のに here.

のに means although, even though, or despite the fact that.

So 病気なのに means:

• even though they are sick
• despite being sick

It often carries a feeling that the situation is unexpected, unfortunate, or contrary to what you would normally expect. In this sentence, it suggests that if someone is sick, you would expect them not to work overtime.

Both 残業をして and 残業して are possible.

• 残業 is a noun meaning overtime
• With する, it becomes to do overtime / to work overtime

So:

• 残業をする = to do overtime
• 残業する = to work overtime

Using を is a little more explicit, because 残業 is treated as the object of する. In everyday Japanese, both versions are common.

していて is the て-form of している.

• している = is doing
• していて = is doing, and...

The て-form connects this part to what comes next. So the sentence flows like:

• even though my friend is sick, they are working overtime, and I feel sorry for them

It links the situation of working overtime to the speaker’s reaction かわいそうです.

Yes. している shows an ongoing action or state.

So:

• 残業をしている = is working overtime / is doing overtime

In this sentence, していて comes from that form. It suggests that the friend is in the situation of working overtime, not just that they worked overtime once in the past.

Because Japanese often omits repeated information when it is already clear.

After 私の友達は, the listener already knows who the sentence is about. So Japanese does not need to repeat 友達が or 友達は again.

English often needs subjects more explicitly, but Japanese frequently leaves them out when they are understood from context.

No, it is completely different from かわいい.

• かわいい = cute
• かわいそう = pitiful, poor thing, I feel sorry for them

In this sentence, かわいそうです means the speaker feels pity or sympathy for the friend.

So the nuance is something like:

• I feel sorry for my friend
• My friend is poor thing
• That’s really sad for my friend

Even though かわいそう and かわいい sound a little similar, they are different words.

It can be, depending on context.

When used about a third person, like in this sentence, it often simply expresses sympathy:

• 病気なのに残業をしていて、かわいそうです = I feel sorry for them because they are working overtime even though they are sick

But if you say かわいそう directly to someone, it can sometimes sound patronizing, as if you are looking down on them.

So here it sounds natural, but learners should be careful about using it directly to a person.

This is normal in Japanese.

Only the final part of the sentence needs to show the overall politeness level. Earlier clauses, especially subordinate clauses, often stay in plain form.

So this combination is very common:

• 病気なのに = plain-style subordinate clause
• 残業をしていて = plain-style connected clause
• かわいそうです = polite ending

This does not sound inconsistent. It is standard Japanese.

By itself, 友達 can mean friend or friends, depending on context.

In this sentence, it will usually be understood as singular because the English meaning given to learners is probably my friend. But grammatically, Japanese does not always mark singular vs. plural the way English does.

If needed, context would make it clear.

That is a very typical Japanese sentence pattern.

Japanese often puts background information first and the main reaction or conclusion at the end:

• 私の友達は = as for my friend
• 病気なのに = even though they are sick
• 残業をしていて = they are working overtime
• かわいそうです = I feel sorry for them

This lets the listener hear the circumstances first and then the speaker’s evaluation at the end.

Yes. A few natural alternatives are:

• 私の友達は病気なのに、残業していてかわいそうです。
• 私の友達は病気なのに残業していて、本当にかわいそうです。
• 病気なのに残業しているなんて、私の友達がかわいそうです。

These all keep the same basic meaning, but the nuance changes slightly. The original sentence is already natural and understandable.