HMRC will regard your friend as having two jobs, with one being as an employee and one as self-employed. He's legally obliged to register his business with HMRC. (That has to be done within 3 months of commencing self-employment, otherwise there's a £100 penalty). Since his full annual tax allowance will be used up within his employed work, he'll have to pay tax (at 20% unless he's earning enough to take him into the upper tax bracket) on every single penny he earns.
If your friend was signing on, he'd need to be aware of his 'signing weeks'. For example, he might need to 'sign on' on alternate Thursdays. That would mean that each 'signing week' would run from Friday to Thursday. If, during any such period, he worked for 16 hours or more, he'd be obliged to 'sign off' (for the relevant period) and then 'sign on' again, with the subsequent loss of benefit.
If your friend was 'signing on' but working for less than £16 hours per week, he'd lose benefit at a rate of £5 less than his earnings. For example, if he earned £40 in a week he'd lose £35 in benefit.
Chris