Her first mistake was consenting to the search. Never consent to a search of any kind. If they have a warrant, let them do their job but don't consent to anything. If they don't have a warrant tell them they need a warrant. If they persist, tell them you do not consent to any search of any kind, but don't try to stop them. You don't have to answer any questions and if they ask you questions, politely tell them you want your attorney present for any questioning.
If they seized a plant, there is an investigation underway of some kind. Whether they come back with a warrant depends on how serious they believe the matter is. They could be obtaining a warrant to return for a more exhaustive search. If they do, they will make a mess of the house but don't be tempted to "assist" them in any way. Just let them do their job. Don't resist them and don't consent to anything. Ask for an attorney.
It cannot be determined from the outline you have provided whether the officers believe the plant belongs to your girlfriend. More specific information would be required. I suggest you contact an attorney for a consultation to determine whether she is likely to be a target of the investigation. The sooner an attorney is on board, the better. Inquiries with the authorities can be made by an attorney without the danger of her making incriminating statements.
They could be screening charges. If she gets an attorney, the attorney can coordinate a surrender and likely a release in order to avoid the embarrassment of officers coming in and taking her into custody without warning.
What your girlfriend is dealing with is very serious. She needs an attorney.