Decativate product quantity field 2 edit 4 certain user role

Hey Freelancers, In Magento 1.4.0.1 I have 3 user groups: admin, management and co-worker. The users in the user group co-worker shouldn't be able to edit the quantity in the QTY field when you go to manage products. Everybody else, as it is currently, are allowed to do it. As of right now, I can only deactivate the whole menu "manage products" for certain user groups. I would like to have to ability to deactivate the QTY field for certain user groups too. Instead of being able to change the number, they just see a gray field with a number in it, without able to edit. I hope I explained it clear enough. Thanks a lot!