As a freelancer, he will see his bottom line increase significantly than were he on the payroll for 2 days.
As a regular employee, sure, he will get a certain number of statutory benefits - the likes of which I have not missed one iota since becoming self employed 5 years ago, the increase in income covers most of these.
Say the company has budgeted X amount to pay an employee per day, a large proportion of that sum will be taken out to cover those 'benefits', and the employee will have to pay tax & NI. The bottom line that the employee gets in their pay packet is X minus a big old wedge.
If he invoices as a sole trader, or better still a limited company, the company he's working for will still pay X per day, but he gets it straight, with no prior deductions and with some careful accounting, he can get a massive reduction in tax & NI. Add onto that registering for VAT and your looking at some hefty savings.
Not only that, the company he's working for will have fewer costs involved in employing a freelancer than they would a regular payroll-employee so they may be more inclined to increase the amount they're willing to pay.
It takes a certain kind of person to feel comfortable with all this, but if you can get past the idea that its less secure, you will soon realise that there's no better security than being in control of your own employment.
All that said, you DO have to make sure your fee reflects that additional risk factor, which is why its important he negotiates!!