Debbie Purdey wants her husband to commit suicide. She went to court because this is currently illegal in the UK, and her husband would be arrested if he helped her.
This is the same whether she wanted to commit suicide because of MS, or because of depression. Legally, there is no difference.
Maybe there are people who want to assist someone else with their suicide and that someone is suicidal through mental illness. You have no idea, and neither do I.
The point people are making is that lots of people with mental illness can, and want to get better. They are then helped to get better, in lots of cases, and go on to live full, enjoyable lives.
Some do not get better, and commit suicide. Were anyone to help them, this would be illegal.
I have tried to be clear. If you continue to be wilfully daft, I may have to take evasive action...