they definitely flex.

lopes link + changing your rear hub from a qr to a bolt on or thru-bolt-qr, definitely helps alot. Check and make sure the lower link bearings are good. They go fairly fast, and a notchy/worn bearing sure seems to add to the flex, in my own testing anyways.

There are something like 14 different revisions of the lopes link, with each newer model helping to add stiffness....or something like that.

The biggest thing most mojo riders notice is the flex seems to cause some deflection mid-turn. It isn't a big deal unless you're really booking it in a really rocky turn. Off-camber doesn't help.

With that said, it is still one of the best (if not THE best) all mountain bike on the market, and he should probably get one

these are just the things I've noticed. Others can feel free to chime in.

 

Yeah, my Mojo used to flex on turns before I got the link, especifically when I changed direction while the suspension was compressed (at the bottom of a slope)

 

I have the original mojo without the lopes link. I ride it relatively hard (I like to think!) gondola riding, I dont really notice much flex. Maybe i am not very observant or its because i am quite light - 60kg on a small mojo.

 

Don't get to hung up on the flex issue. Not a big deal. It's not like its a swing bike (Google it) or anything.

 

I agree dont get hung up on the flex, awesome bike to ride.

 

My 07 Mojo with a Lopes Link is about as flexy as most full suspension on the market.

 

After buying a used std Mojo a while back, enjoying it, upgrading it, and generally riding the snot out of it... then upgrading to a new SL and changing all the links/bearings myself, I would suggest that your friend budget for new upper & lower links in his purchase. The stiffness benefit of the lopes link is well known. The lower link is the one that's gone through many updates, so there's an improvement to benefit from there too... and then just the simple fact of having brand new bearings is worth it. I loved my bike before upgrading the lower link, but when I pulled the original off I was amazed at how notchy the bearings were....it's not always apparent on a built up bike.
Plus, he can pick a fancy-schmancy new link color and exorcise any demons hanging around from the previous owner.
The SL is plenty stiff for fast, aggressive, rocky riding under this 160 pounder.

 

The wheels you roll with also make a huge difference in rear flex. I have a superlight wheelset I run when racing or if I'm on a long cross country trail, and a superstiff, DH wheelset I run for everything else. The superlight wheelset is way flexier than the DH wheelset.