need help w/friend's turbo problems

i have no idea what is going on, but he says that when he hits boost and nails the gas the car starts to lag and stall, and the A/F guage bounces from lean to rich when he's off boost and on boost, what could be going on?

What kind of car/setup?

The A/F gage is suposed to go back and forth until the car is WOT.

Possible boost/vacuum leak or maybe the car is sensoring knock and pulling the timing...

Couple ideas at least.. Dont know if that helps..

Ryan