Also I have a brand new copy of Windows 7 Professional still in the box and unused can he use it to update from HomePremuim to Professional.
Yes he can use it to go from Home Premium to Professional, but you'll have to sell it to him. Also, he COULD do an upgrade install but a clean install will be best.
Why does he want to go to Pro? Not that many people have use for the extra features of Pro.
If you let him borrow it to do this, he'll use the product key for it and then you'll NEVER be able to use it for yourself. Only ONE install per product key is allowed. So if you want to sell it to him that's your choice.
He can do an upgrade via Microsoft's Upgrade Anytime feature in the Properties section of the Computer icon. Right-click on the Computer Icon and go to Properties listed at the bottom of the pop-up windows. Click on that. Then click on the: Get More Features With A New Edition Of Windows 7 highlited in blue.