I do this every couple of weeks and delay depends on time of arrival. If she's on the UA 901 from FRA she should be out in less than 20 min. If she's coming on the later flight it could get a bit busy but I have never spent more than 20 min in passport control. Ask her to try sit closer to the door of the plane so she's relatively ahead in line. My personal record is 5 min from touchdown to customs (first out of the plane and no line at passport control).

Make sure to account for time taken to clear security when she returns. this is where a train would be a better option so you're not stuck in traffic.