To ask who is right here?

[Soletraderlady]

I am setting up as a sole trader. I thought I had to inform HRMC immediately or in the next 3 months otherwise I will face a fine. Friend said I can wait until I hit the £1,000 threshold. Who is right?

From gov.uk:

You need to set up as a sole trader if any of the following apply:

you earned more than £1,000 from self-employment between 6 April 2020 and 5 April 2021
you need to prove you’re self-employed, for example to claim Tax-Free Childcare
you want to make voluntary Class 2 National Insurance payments to help you qualify for benefits

I'm sure I read that it has to be within 3 months regardless of what you have earnt? So even if it was just £100 you would need to tell HMRC.