Right before she kisses him in an attempt to revive him, she removes her glasses. Before the password brings John here to make an alternate timeline as seen in the recent updates, Gamzee takes her glasses and uses them to send another message to Karkat, through her own account. This occurs in Karkat: Answer Terezi, and after Karkat has a small breakdown over Sollux, he messages Equius and asks him to go after Gamzee. This sparks Equius: Seek the highbl00d, during which Gamzee kills Equius and subsequently kills Nepeta.
Everything after that - right down to this shifting of events - stemmed from Gamzee stealing the glasses and causing a chain of deaths that only less than half of the trolls survived(and in Sollux’s case, not even long enough to do much more than fire the meteor towards the new universe).
Now, however, Terezi was interrupted by the witness.
She put her glasses back on - BEFORE Gamzee can steal them.
Now, when Karkat messages her while he’s with Sollux, she can answer him.
This changes a lot of things;
1. Gamzee won’t have Terezi’s glasses to scare Karkat, and even if he ordered Equius after Gamzee, he wouldn’t have the glasses on to momentarily trick Equius into thinking it’s Terezi(see first image)
2. Because he will not be ordered to fight Gamzee, Equius will still be with Nepeta by the horn pile. What will happen to them is unknown.
3. Terezi will have her glasses to respond to Karkat, and they will discuss, as Karkat puts it, “GETTING THE FUCK OUT OF HERE.” This will likely prevent the escape and/or death of Vriska.
In essence, it wasn’t necessarily the witness himself that changed the timeline; it was Terezi’s glasses. There are probably other things that this changed, but the big one is that in this edited timeline, Terezi(likely) stayed on the roof longer, Karkat didn’t utterly panic and send off Equius, and Gamzee never scared anyone with the glasses.
