I am not a professional investigator but I would think that you would assess the level of danger by determining one by one if the woman's claims were true. If they appear to be true, then the threat level would be high and immediate action taken. First, I would try to get a voiceprint spectrogram match between the man in the woman's recordings and a known recording of the man's voice to see if there is a match. Second, I would try to get any surveillance footage from businesses and buildings in the area of her work to see if the man can be identified as being documented in the area at the time of the threatening calls. You could also check credit/bank card records to see if there is a transaction at a nearby business at the times in question as well. That would place him at the scene at the time of the calls. Finally, there is the matter of the slashed tires. This could be hard to find direct evidence of since cars are parked in the open and and trace hairs or fibers from the person can easily be lost from the scene. You could try fingerprinting the car near the tires but its doubtful you could get useful prints from the outside of a car. Probably the best thing is to have the woman park in an area that us under video surveillance and see if he can be caught in the act again.