I’m also just guessing as I don’t know the laws in your area but…
The only thing I would assume they could get you for is loitering. You can’t really trespass in a multi-story carpark as it’s made for the public to enter and park there at least during operating hours. Most security guards that I know have no real authority and can only detain you (nonphysically) to wait for the cops who can arrest you on whatever charges the security guard has. Most security guards are there almost as a moving video camera really. They observe the grounds as an official to deter petty criminals. (ie, skateboarders, graffiti artists, petty thieves, ect.)
Now, I’m pretty sure most carry some form of self defense such as pepper spay or even knight sticks and they can defend themselves if you so choose to put up a fight. Other than that, I don’t think they have much authority.
That being said, some companies hire ex-military or ask the police to have a squad car drive around the area every so often and if you run or put up a fight against them, you’re looking at some possible serious trouble.
Again, I don’t know the laws or regulations in your area so…. I’m just going off what I know about the few security guards in my area.
If I were in your shoes, I’d tell your friend to forget about the board and stay away from there for a while. If you’re gonna be a rebellious skater, you have to be ready to pay the piper at some point. I’d run from security… just not cops.