The thing that sounds strangest to me is the part about a detective calling her house on the phone. Why would they tip her off? If they're going to arrest people, I'd think that they would just come and do it. Usually, there is a buy-and-bust type of scene, or they search your dwelling and person. Sounds fishy, like it's a ripoff artist, or a hoax. The people that sent text messages requesting weed probably have nothing to worry about in the event that it is a real investigation. Maybe they'll be pushed to be witnesses, in which case I would recommend a lawyer. This is all just speculation, as I've never been arrested or had any significant legal problems with weed.

Basically, if there has been nothing confiscated from her, and she hasn't been caught in the act of committing a crime, it probably isn't serious.

Like S.W.I.M. 504 said, just deny everything. If arrested, which is unlikely, don't say anything without an attorney present. They can't put somebody away for reasons solely based on text messages about weed.