Absolutely ridiculous and bloody stupid. Not only could HMRC look into it but also the bank themselfs. They have a duty to monitor and flag any suspicious activity. If you’re banking cash into an account, you’re at risk of the bank asking questions. So there’s one issue.
If HMRC do investigate this person, they have made it very obvious that they are trying to avoid tax. A huge NO NO! And very serious.
There are much better ways of reducing your tax. Like claiming a lot of expenses, even if some aren’t 100% business only, you can push your luck and the worse that is going to happen if HMRC investigate you, you play dumb and act like you thought you could claim XX expenses and they will just recalculate your tax bill and you’ll have to pay it with a slap on the wrist
But having two separate banks, one for ‘hiding’ income just makes it way too obvious and HMRC will not be happy and you won’t get away lightly
This friend obviously isn’t very savvy so I’d recommend they get a decent account who’s happy to push the boundaries as much as possible