A good reason not to have a grow partner!! But it shouldn't hurt the plants......certain people harvest the top half and leave the rest for a second harvest. I actually think you should be taking samples as you near harvest....but they should be properly marked, dried and cured before you smoke them. This way you know if you prefer week 6, week 7, week 8, etc.... Everyone has a different preference, so the sample will give you a real comparision...rather than looking at trichs, hairs or whatever method you choose...
I can harvest at 6 weeks but the buzz isn't suitable to me, I didn't try 7 weeks as I use a two week rotation, but I found 8 weeks suits me just fine...........I don't see much difference in flavor, but do see a big difference in bud density, so keep that in mind as you try your samples.......If it tastes the same and the high's the same....then you'd want the most density...but if there's a flavor/buzz combo that you prefer, you might sacrifice the extra weight. That being said......sounds like dude is just feeding his Jones and robbing you of buds. Flowering time shouldn't be extended unless he's snatching top colas and leaving the small stuff......in which case maybe you should kick his ass and/or move the plants from his reach....... and never grow with him again....... If you aren't all sharing the samples............he's self centered and I'd worry the whole plant will suddenly become HIS and the much deserved ass whipping that follows will just land you in legal trouble. You're stuck between a rock and a hard place...my friend. Talk to the other partner.....maybe you should agree that the next bud he takes without sharing.....you'll harvest and divide, taking some of his share to replace what he's taken and be done with him....Not sure what else to suggest as it already seems like a no-win situation. Many a partnership has ended with one guy clearing out the bank accounts and splitting, leaving the other guy broke and in debt.......
Best of luck........