After how long is the car running when this happens? What exactly goes on during this "weird idle"? Does it lose power, shake a little, etc.????

In terms of the mechanic telling her that she needs a new timing belt, that may be so if the belt looks to be pretty worn or mileage wise she is due. If that is the case then she should get her water pump replcaced along with her alternator and a/c belts too.