It depends what you mean by "help". Some things are OK, some are not.
Providing factual information is OK. Saying that you understand and/or respect their decision is OK. Being there with them when they ctb, either in person, by phone, or online, so they don't die alone is OK, so long as you don't do or say anything that would push them towards catching the bus.
Providing something they will use to ctb may not be OK (though it might depend on what the "something" was; if it's something they could easily buy at the local store themselves, and which is only peripheral to their ctb method, it might be hard to argue that you are doing anything improper). Buying a gun for them, for example, or giving them a bottle of SN, would certainly not be OK. But I think it would be hard to argue that buying an empty bottle they can use to store their SN is improper. Saying things like "Yes, you should do it" would not be OK (even if your own private opinion is that they have made the right choice). Actively helping them when they ctb, such as pulling the trigger of a gun, or pushing them off a tall building because they are too scared to jump, would definitely not be OK.
I think a good guideline is the two lines they used to teach to medical students. (Perhaps they still do.)
Thou shalt not kill; but need'st not strive
Officiously to keep alive