Which antitheft device are you referring to? Is this a factory alarm system or an aftermarket add-on? Can you post a photograph of the key fob or any security warning lights on the panel so that I can determine which system you have?
If this is a factory installed alarms system with the immobilizer chip built into the key when you insert the key into the ignition switch the factory light should go out. If it continues to flash there may be an issue with the immobilizer chip in the key or in the immobilizer sensing ring around the ignition lock cylinder. Without seeing the light that is flashing it is difficult to say for certain what system you have. If she has more than one key or a bunch of rings around ahead of the key it can interfere with the signal of the immobilizer chip to the transponder ECU. Ask her what happened when she turned the ignition key did the engine crank and crank and not start or did not crank at all?
If the alarm never sounded this may be something as simple as a starter issue. It is possible that the contacts in the starter are worn out. Did she hear any click or clunk sounds coming from the engine compartment or under the hood when turning the key to the start position?
Unfortunately without seeing the blinking security light or being able to tell if there was any clicking noise it's hard to say from my position behind his keyboard whether or not there is a starter issue of whether there is a security alarm system issue. Let me know if she is able to post any photographs of the security light or any wireless transmitters on the keys that she might have. If this happens again have her cycle the key from off to on to start quickly several times in a row and see if she can get the starter to engage.