I have to agree with AO. You shouldn't criple the player so much for the feat. Now because he wants a character that you may not normally allow in your campaign, that's a bit of your own fault. Would you allow a player to play a character of that class? If so, then why are you cripling a cohort? If not, then why are you allowing a NPC of such?
But in either case your allowing it. Maybe limit how much access you allow at any one time and always only allow her to control it. She can't give it or loan it to others, because only she has the mentality to use it.
Also rather than having her be a sociopath and killing party members and such, have her be someone who really is someone who just is unstable and needs help, mentally and physically. This way the character has to take care of the character and watch out for her rather than worry about her stabbing them in the back, which in my opinion a cohort should never do.
Now the cohort may be very jelous of other party members, but because of her lack of physical capabilities can't really present much of a threat except maybe scratching or telling lies to other non-party members, which may cause enough trouble, but again nothing that would get her boss in trouble, because again that goes against the nature of the leadership feat.
But in either case your allowing it. Maybe limit how much access you allow at any one time and always only allow her to control it. She can't give it or loan it to others, because only she has the mentality to use it.
Also rather than having her be a sociopath and killing party members and such, have her be someone who really is someone who just is unstable and needs help, mentally and physically. This way the character has to take care of the character and watch out for her rather than worry about her stabbing them in the back, which in my opinion a cohort should never do.
Now the cohort may be very jelous of other party members, but because of her lack of physical capabilities can't really present much of a threat except maybe scratching or telling lies to other non-party members, which may cause enough trouble, but again nothing that would get her boss in trouble, because again that goes against the nature of the leadership feat.