You can provide factual information. You can provide emotional support. You can make suggestions about how people could deal with their problems. You can discourage someone from catching the bus. (I have often done that when someone seems to be acting impulsively, or when someone seems to be over-reacting to some problem and ctb is too drastic a remedy.) If someone has decided to ctb, you can say some kind words to them.
You can't encourage someone to ctb, or tell someone that they should ctb. You can't tell them what method they should use if they do decide to ctb.
I think the above covers most of the situations that arise. It doesn't seem difficult to me.
What it comes down to is that if someone does decide to ctb, that decision must be 100% their decision. Likewise, what method they use must be 100% their decision. That seems to me to be the only ethical way to proceed. (It also ensures that you remain within the law.)