[Designated Driver] Driving is truly terrifying

I was driving at 120–130 km/h in the first lane of the highway when the customer said,
“How many kilometers is this road.. It's scary. I don't want to die..”
“Oh, is that so? I will slow down a bit. Would 100 km/h be alright?”
“Yes.”
Then I changed to the second lane.
A moment later, there was a merging lane and I was driving at 90–100 km/h,
but a vehicle from the merging lane kept pushing its way in. (I checked via the rearview mirror)
Just before we collided, I jerked the steering wheel to the side.
The car swerved and skidded just like in a movie, but I focused and brought it back onto its normal path.
When I checked the rearview mirror, it was almost a direct hit, so I steered based on an instantaneous judgment.
The customer
started swearing, saying, “F***, what the hell was that?”..
I told them, “It wasn't my fault. I avoided an accident,”
turned on the hazard lights, opened the window, gave hand signals, and pulled over to the shoulder, and that vehicle followed us.
When we got out to check, it was a very young person.
They looked to be in their late 20s or early 30s.
They started by apologizing immediately.
They said they didn't see me. They kept apologizing with deep 90-degree bows.
It was acknowledged that if I hadn't avoided it, it would have been a 100% collision.
It was truly a split second, and they pushed in until there were almost only 30cm left before impact.
Since I am the designated driver and the vehicle owner is the customer,
I told them, “The two of you should exchange contact information first.”
The customer and I suspected they might have been drinking, but it turns out they weren't.
Fortunately, no one was injured, and since there might be issues with the vehicle later, I told them to resolve it by exchanging contact info and wrapped things up.
But the timing is just...
Of all things, this problem happened right after the customer said those words and I changed lanes.
If they hadn't said anything at all, this wouldn't have happened..
And driving under 100 km/h among freight trucks actually felt more dangerous.
I always have a habit of checking the rearview mirror while driving, and after experiencing this, I felt once again that it is a truly good habit.
If I hadn't avoided it then and had just hit them, the car would have definitely overturned. Because we would have struck the rear passenger side directly...