Hmm, A deca only cycle, not such a good idea! This is my personal opinion, so anyone feel free to discuss. A better first cycle would be to run test for 8 - 10 weeks, 500mgs per week, then PCT.
If you're running Deca, then you should run it with Test at a higher dosage, eg 500mgs test per week, 300 mgs Deca per week.
Because of the differing half lifes, the test should be run 2 weeks longer than the Deca, eg Deca @ 300mgs weeks 1 - 12, Test @ 500mgs weeks 1 - 14 so they both clear your system at a similar time before starting PCT
I wouldn't use Deca without using HCG in my PCT. Deca shuts me down bad, so I personally would probably run HCG throughout my cycle if I ever run Deca again to help recovery.
Deca for 5 or 6 weeks is a waste of time. I doubt your friend get any good results from this cycle. I know people who have run deca only cycles due to the 'low side effects' of this drug. Some of them were OK, but one guy was stuck with Deca Dick for months and wasn't a happy man. Its your friends choice, but I'd stack it with test and never run it on its own. This is a bad cycle and won't yield results