A lot of it gets buried under the music, but her line on the phone is
"What did you do Kenny? They say it's kids; that you've been looking
at kids. & Lucy saw it, it was a video of you. You're doing
[indistinguishable] & all her friends have got it. Kids Kenny."
The rest is less distinct, but it doesn't mean she saw what he was looking at, nor even the act itself.
Going by what she said, she's only heard it reported.
That does, of course, simply remove the problem to one remote - if his mother didn't see it, who actually did & what did they actually see... somebody either saw
- him and what he saw, from an undisclosed camera [unlikely]
- only a report of what happened, or
- the antagonist posted both, the web-cam video & screen caps etc from his computer
That the police turn up mob-handed mere seconds later makes for good drama, but we have to assume they wouldn't come out in such numbers for mere hearsay. They also don't know at that point that he'd killed the guy in the woods, unless the drone footage was also sent to them along with the other evidence.
Late thought - they knew where he was, so they probably have seen the drone footage too.
If that's correct & the police did have the drone footage, then whether or not he was actually watching child porn, by that time to the police, is probably almost moot. They're coming to arrest a killer.
The only 'direct evidence' we do see is the CEO 'racist email' scandal on her screen, with a newspaper report & excerpt from one of the emails.
I think we're left to assume similar details were provided for the other 'victims' - so whether it's a true plot hole or not, I'm not really sure.