I don't know what the US law is, but in the UK at least even flying to Dignitas (for example) with someone is legally not kosher. I didn't pry but I think this situation lead to a suicide in my family also (attributing a single cause to a suicide is usually not a good idea so I won't ponder, other than that it wasn't clean or dignified.)
I think the same argument can be made for terminal illnesses also but I think the issue there is extreme pain rather than loss of mental faculties which is potentially not the same thing in the eyes of a court.