where have all the boundaries gone?

my mom has bi-polar disorder. this causes her to make poor decisions and to fall into serious bouts of depression. she can't help it - it's biological. but that doesn't make her actions any less frustrating. so my question is: where does one draw the line with people with mental illnesses?

i have a friend whose husband is very obssessive-compulsive. i doubt he'd get a clinical diagnosis, but it's bad enough to send my friend into a major rage when he gets "that way." for example, she had to be somewhere early yesterday, but her other half was so wrapped up in the details of packing their son's lunch that she was late. she called me in a fury saying that she has had it with the o-c behavior. but he can't help it. what to do?

my mom's friend called me the other day, upset because my mom had suddenly given her the cold shoulder and wouldn't return her calls, etc. when this happens to me, i shrug it off. i know she gets that way. i explained that this has happened to me on and off for my entire life and not to take it personally. but what happens when my mom really goes too far? i suppose a frank conversation with her when she's back in functional mode may help, but then again, it may not. it's like being mad at a baby for pooping in her diaper - she can't control it. but does that take away one's right to be mad?