I want to tell my friend not to get another cat
Why don't you want her to have another cat? Are you allergic to them?
I don't think they could handle another one.
If your friend wants another cat and they can responsibly take care of it, you shouldn't try to stop them. But if you think they can't reasonably take care of another pet, you should do as you say, and try to talk it over.
I'm going to tell her while she's drunk to not get another.
If your friend has already made up her mind to adopt this other cat, you shouldn't be resorting to trickery to get her to do what you want. However, if you have a reasonable objection, you should bring the subject up and talk it over with your friend.