While she was holding the Play and Fast-Forward buttons, we are explicitly shown that Marta hits the Eject button. This is when the tape ejects and the player starts smoking, which the security guard blames on the VCR. That act doesn't necessarily damage the tape in any way, but Marta takes the tape out and holds onto it, and we see later that she also takes a fridge magnet from the security room. After Wagner takes the tape from her, we see her put the fridge magnet back into her coat, implying that she used it to wipe the footage (The security guard indicated in the prior scene that a magnet would destroy the tape footage).