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he’s only telling it how he sees it.

Could you please tell me what's the meaning of the sentence above?

Is there any idiom here?

Note: The question has rewritten. So the(colleenv)'s comment refers to the original version of question.

The full text is:

Beverly listens to him, licks her lips nervously, and waits. No one else
volunteers anything.
Finally, she blurts out, “There’s something I should probably say.”
All eyes turn her way. She almost loses courage. She doesn’t know if the
argument between Dana and Matthew is relevant or not, but it will certainly
sound damning.
“What is it?” David says calmly, as she hesitates.
“I heard them arguing, last night.”
“Dana and Matthew?” David says, as if in surprise.
“Yes.”
“What was the argument about, do you know?”
She shakes her head. “I heard them shouting, but I couldn’t make out any
words. Their room is next to ours, on the same side of the hall.” She looks at
her husband. “Henry slept through it all.”
“What time was this?”
“I don’t know, but late.”
“Did it sound . . . violent?” David asks.
“I don’t know. It was just raised voices. No crying or anything. Nothing
slamming, if that’s what you mean.”
There, she’s said it. If Matthew’s done something wrong, then it’s good that
she’s told them.
David can sense the heightened distress of the others. They don’t like
what Beverly has said; it makes them uneasy. They don’t like to think
the unthinkable. He can see from their faces that they are all
imagining it—the argument, the push down the stairs. He’s sorry for
their distress, but he’s only telling it how he sees it. It doesn’t
seem possible that Dana could have been injured like that from her
fall, and he doesn’t want them messing about with the body. And now
this new information—Matthew had told him that he and Dana had not
argued. If Beverly is to be believed, Matthew lied to him.

A referee at a football match calls plays as he sees them. That is, as he observes and makes judgments and comes to an opinion about the situation.

If you describe a situation in terms that someone else might consider unflattering, and they criticize you for doing so, you might reply:

Look, I'm only telling it as I see it. I'm not trying to make it out to be any worse or any better than it seems to me.

A person who tells it as they see it is not trying to distort the situation as someone might who has a bias or an "agenda". They're trying to be objective as possible, given the limitations of their vantage point.

Dictionary.com covers this definition of see:

5. 
   

   to perceive things mentally; discern; understand:

   
   
   

   to see the point of an argument.

   
   

When he's telling it how he sees it, he's conveying his thoughts relating to Dana's murder.

There's a common idiom in English that is similar to your sentence (maybe it is being confused in the discussion of idioms) called telling it like it is, or describing a situation truthfully with no regard for pleasantness to the listener.

In this case, the use of the word only implies that David is not intentionally trying to upset the other people; as a detective (I'm guessing), his job is to investigate the scene and find as much information as possible that will solve the case, including a cause (domestic abuse).

Yes, there is an idiom.

The usual idiom in speech is: to tell it [whatever the thing is] like it is.

tell it how he sees it, is a variation on that.

The fact it is present progressive refers to a single instance of this in the story. The rest of the paragraph is also in the present tense.

If this were all the time, one would say:

He tells it like it is. [every time he tells the story, for example].

Simple present for verbs like this mean a general statement, not a specific one.