If his buds have been in cure for 21 days and still smell like grass, then i think the problem is more with the drying process prior to going into the glass jars, than the flush. Flushing is absolutely important when you are talking about taste, and how the buds smoke, but as far as smell goes and how they cure, personally i dont think it makes much of a difference. Ive had some SUPER CHEMMY bud that wasnt flushed properly, but it smelled like an absolute dream in the bag. Ive had fully organic, properly flushed nuggets that looked amazing and smelled like absolute ass -- almost on the verge of mouldy. Its all about the drying process prior to the cure.
You HAVE TO make sure that the stems snap and the outside of the nuggets are crispy before you take them down from hang drying. After that, a couple of days in and out of paper bags/cardboard boxes will sweat out any residual moisture. This NEEDS to be done in order to have a bud cure properly. If there is too much moisture on the inside of a bud when you put it into glass, it will basically get locked in the flavor and smell after a mere 1/2 a day in the jar. Sure you can take it back out and dry it some more, but itll still smell like ass, at least with my own personal experience man, lol. My first grow was a disaster on curing. I made a bud dryer, and let them hang out in there for 3 days. The buds werent anywhere near done. I let them breathe a bit in paper bags, and they were just starting to get dry on the outside, when i put them into jars.
Within one day, i woke up the next morning (i burped the jars like i was supposed to for the first day), and opened up my jars to check them out, and it smelled like rotting grass. No matter what i did, drying them again helped a bit, but i could never really get rid of that smell.
Anyways, just sharing my own experiences with you man. I really truly do hope im wrong though for your sake. It would suck to have some stellar bud that got you high as shit, and looks amazing, but the smell just puts you off... I hope that you wake up one morning and end up with some absolutely Gourmet nuggets in your jar man.
Good luck !
-K1.