Personally, as far as this case goes, I think there was really no option other than to arrest him. Say what we will about our right to die, and the place of a family member in helping us along, but really look critically at this case and I can't help but think they needed to investigate if that's even what happened in the first place. I hate to say it but there could be a dozen different variables that made this homicide, not assisted suicide.
I think that, until we have a system in place for end of life assistance (and I really believe we should have something for people that are suffering and have no real hope of recovery), we need to be arresting/investigating all instances. It's too easy to use that as a excuse, even if the spouse/partner has a terminal illness.
I saw a program a few weeks ago on ID Discovry about a woman who killed her husband who had advanced terminal cancer. He was going to die anyway, no two ways about it, but I guess it wasn't fast enough for her. She fed him, I can't recall if it was poison or an overdose of his more potent meds, and when he didn't expire quickly enough, she eventually stabbed him. Now obviously it would be really hard to claim mercy killing in her situation. But, had she been more patient and let the meds kick in, she could have had a reasonable argument/defense should they find out he died from an overdose.