Yep, agreed Sammy, however all he knows at this stage is that there is smoke coming out of his pipes and there's oil in one cylinder. A compression test will tell him the condition of all 4 cylinders, and if it's only one cylinder that is causing the problem, then he can concentrate on that one.
It could be a knackered piston and/or rings, or a burned valve, and once again, there are tests that he can do to determine which problem he is likely facing. A worn valve guide will make a bit of smoke, (moreso when he backs off than on accelleration) but I doubt that's going to reduce the power to almost nothing.
I bought a compression gauge on EBay for under 20 bucks, and there are plenty available on EBay now for that kind of money, so it's no great expense, and a great diagnostic tool. If his engine needs a complete top end rebuild, it won't add a great deal to the overall cost. Also, it'll be very handy for him to take with him next time he goes to look at a bike that he's thinking of buying. It'll probably save him a lot more than it's purchase price.