Upvote:2
They will most likely have to leave the US by the 90th day after their first arrival. In other words, their second stay will most likely be limited to a month and a half.
See the State Department's web site (https://travel.state.gov/content/visas/en/visit/visa-waiver-program.html):
Trips to Canada, Mexico, or nearby Islands
If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less. See the CBP website. Citizens of VWP countries* who reside in Mexico, Canada, or a nearby island are generally exempted from the requirement to show onward travel to another country* when entering the United States.