If it's repetitive, and has happened on multiple frames, I can't see this problem happening strictly from racquet abuse, especially if the player is not abusive on his frames. If I was forced to guess it'd be from poor mounting. OP, can you check the frame shape against an identical racquet that has never been strung by you NOR the other stringer that the player has dealt with? Do you see any rounding of the hoop? I'd measure the frames and make sure they're at spec. Repeated breakage indicates a systematic shortcoming, to me.
As far as which machine you should be using -- it shouldn't matter too much, although in this case, it'd be safer to use the 6 pt. The NEOS mounting system is fine -- and is adequate, but there is a lack of support further from the 6/12 positions, and you WILL see flexing on flexier frames (Head Prestige MPs comes to mind). On a 6 point machine, you should be adjusting the mounting stocks based on the 6/12 position first, and you should get a finger tight (not too firm) mount. Once you've done this, get a finger tight mount on the support mounts. You should NOT be torquing down the mounts too hard. These should be only finger tight. I'd use your pinky and thumb to tighten down to get a feel. (In case you aren't familiar with mounting).