At what point at work do you say this person is completely useless and you get rid of them?

We have someone in the team since the new year.

It's normal to take a long time to get used to everything. anyway this person appears to be getting worse. They have tried everything to help. They have stopped giving them new stuff to do.

It's not that the person is not trying they are staying 2 hours every night extra to catch up. they are just a mess. Now the boss has said you can take a week off holidays if you like as your are stressed out.

I don't want to see anybody get rid of but this person is uselss and it's making everyone else job more difficult.

I noticed that this person even has someone else's email address in their email signature as they've just copied and pasted the signature from somebody else.

They don't seem thick when I speak to them. the boss spends several hours a day going through all their work one thing at a time.

Very strange.