Disclaimer: anything I know about this person's financial situation comes from information they have shared with me directly (not speculation).
I have a family member who has been running a business for four years without declaring it as income - about £800-1000 a month, so below the tax threshold I think. Her partner has relatively low wages, so they claim a substantial amount in tax credits each month. She's told me and other family members that she has no intention to declare her earnings because their overall income would drop significantly. And because of various other justifications (it's nothing compared to corporate tax avoidance etc). Her partner works the occasional cash in hand weekend job, but it's not as sustained as this. They have six children.
They're not in a financially difficult situation. Partly because they haven't had a mortgage/rent to pay for the last decade, they've been able to put aside about 50k in savings which they've used to buy a larger house for their family, with extension planned etc. They're not wealthy, but they're certainly not struggling.
What would you do? Upfront, I have no intention of reporting this, sabotaging the family relationship or just making things difficult for her children. But I'd like to know what the real risks are of actually being caught by HMRC, and the implications. If so I would seriously warn her of it (although she is incredibly stubborn and probably wouldn't listen). Or would you just stay out of it completely?