I think you are definitely within your rights to have that discussion.
What will your next move be when/if she refuses? She will have to make the arrangements to go to _ _ _ _ _ _What kind of treatment? For how long? Afterwards, what kind of follow-up care would be agreeable to you?
You only have a couple aces to play here. If she won't get help, will you continue in this relationship?
As you can see, "You need treatment," is just a statement that she can and likely will ignore/refuse. Then what?