Hi, speranza, and welcome!
You're right to be concerned. Your friend has become involved with a destructive group...and her obsession with it is a symptom of what they're doing to her mind right now, I'm afraid.
If possible, I think the best thing to do is to stick by your friend, stay supportive, remind her of good times you had together before Landmark, and try to read up about Landmark as much as possible. Landmark will try to separate people from loved ones who are not interested in Landmark -- it helps make their victims less independent and more vulnerable to their manipulations.
It's true that your friend can't hear anything negative about Landmark right now, because she's on a "high." Anything you say about them being a "cult" will seem to her like a personal attack (that's what she's been coached to believe).
There's information here about coping:
And lots of information about Landmark:
Also, I've read that although some people won't listen to negative info about their own group (in this case, Landmark), it may be possible to get them to read info about a similar group. Several people have found this study, about Lifespring (a group very similar to Landmark) very helpful:
This study gave me a fairly clear idea of what actually happens in these types of "seminars."
Sorry your friend has fallen victim to Landmark...I have a friend in the same situation, which is why I'm here.