The visa is still valid till January 2023. Can the employer cancel the visa online or by using a photocopy of my friend's passport?

Assuming that you actually mean an extension for working and not an actual visa:

The employer can't cancel the extension. But the day the job ends the extension is void.

If the employer does nothing and the ex-employee will just leave Thailand while the stamp in his passport is still valid, then immigration will just let him leave and that's it, because immigration isn't aware that the extension actually became void.

If the employer wants to make troubles for his ex-employee he could of course inform immigration that the work ended and that this person is now on overstay. In his case the person might be arrested for being on overstay.

17 minutes ago, FriendlyFarang said:

Assuming that you actually mean an extension for working and not an actual visa:

The employer can't cancel the extension. But the day the job ends the extension is void.

If the employer does nothing and the ex-employee will just leave Thailand while the stamp in his passport is still valid, then immigration will just let him leave and that's it, because immigration isn't aware that the extension actually became void.

If the employer wants to make troubles for his ex-employee he could of course inform immigration that the work ended and that this person is now on overstay. In his case the person might be arrested for being on overstay.

Was there an article reading that if one simply got another  job that visa would be valid still?  Seems like I read that somewhere,  but I could be mistaken!

A visa cannot be canceled but a extension of stay can be canceled by immigration.

Normally immigration will not cancel an extension without having the passport that it is in, A employer cannot cancel it.