Learn English – reply to “How are you doing?” with “I’m good”

You are right; It is odd. And I don't think the characters do a good job with this mini-scene. From the video clip:

A: How's Nando?

B: I (don't) think so.

A: Well, maybe it's just as well; his mother's dead.

This exchange comes in the very specific context of checking for survivors just after an airplane crash in the Andes Mountains. This is why we almost always ask for context.

Frankly, after viewing the clip and reading about the plot of the movie, for which the real-life Nando is a technical adviser, I still don't grasp exactly what B replies and what A's statement means.

By saying "I (don't) think so" in a very uncertain manner, shifting his eyes around a lot, B could be saying a number of things:

a. I don't think he's alive.
b. I don't think he's dead.
c. I don't think he's going to survive.
d. I don't think he's going to die.
e. He's unconscious and I don't think he's going to survive.
f. He's unconscious and I don't think he's going to die.

Therefore..., I am not even going to attempt to guess why A says whet he says about the mother being dead. :)

Original Answer:
The question and answer you have reported indeed do not sound typical. But I doubt the actor made a mistake that was not caught by the director or film editor.

You have supplied only two sentences of dialog, and we have no other context to go on. We do not even know what movie this is from. And you have not told whether the person on the ground is okay or not. We also cannot see the gestures, facial expressions, and/or body language of the two speakers. We also know nothing about the relationship between the two characters (Haram and Linda). We also do not know anything about the individual speech characteristics of either character. Any or all of these things might affect how Haram responds to Lisa's question. Language is only one way to communicate and does not happen in a vacuum.

Having said all that, there are some reasons Haram might have said what he did, but these are only guesses, since I have none of the above information.

1. The person on the ground may be dead or seriously wounded, so the answer I don't think so could mean Don't bother to ask that question, it is irrelevant.

2. The relationship between Haram and Linda may have some stress in it, so Haram may mean I am not talking to you. Or I am not talking to you until we discuss our relationship.

These are only two possibilities which demonstrate how 'extra linguistic' elements may contribute to how someone responds to the question. Or it could be something 'silly' such as Haram likes that phrase and says it all the time.

We really can't answer your question until we have more information.

Last, you mention that 'Haram finds a person who lies down on the street.' This means Haram first saw the person in some other position (standing or sitting) and then the person lies down on the street.

The sentence 'Haram finds a person lying on the street' would mean the person was already lying on the street when Haram first sees the person.

I think what the page meant by saying "when, where, who" is that the perfect form doesn't go well with expressions suggests a particular time. The example sentences given at the linked page seem to be all about it.

The default past tense is the simple past, as Michael Swan says in his Practical English Usage (§421.4 "In general, the simple past tense is the ‘normal’ one for talking about the past; we use it if we do not have a special reason for using one of the other tenses.")

The perfect forms are needed basically to add the sense of completion to non-finite verbs such as infinitives, participles, and modal verbs. Because such tool exists, it's also used to talk about past events, but it ends up saying 'up until now' ('up until sometime ago' when it's the past perfect). Because it's tenseless by itself, it makes the sense of time vague, thus it has the sense of duration at the same time it conveys the sense of completion.

The use of the perfect forms are more to do with meaning than tense. Grammar books usually says it's one of tense form, but actually it's not about tense. Linguists call it 'aspect'.

The perfect form goes well with expressions like already, recently, just, since, ever, never. But it doesn't go well with expressions of particular point of time, when, such as yesterday.

I've read something more relevant to the OP's question, in the same M. Swan's PEU:

§457.1 (...) we usually prefer a past tense when we identify the person, thing or circumstances responsible for a present situation (because we are thinking about the past cause, not the present result). Compare:
Look what John's given me! (thinking about the gift)
Who gave you that? (thinking about the past action of giving)

Also

PEU §456.5
We normally use the present perfect to announce news. But when we give more details, we usually change to a past tense.
There has been a plane crash near Bristol. Witnesses say that there was an explosion as the aircraft was taking off, ...