Some friends flew into the US (LAX) under the VWP and drove into Tijuana, Mexico. They will be in Mexico for 2 months before flying to El Salvador, Costa Rica, and Colombia for a month and then back to Mexico.
They are booked to fly out from LAX 96 days after first arriving, and will be driving back to Los Angeles from Tijuana on that last day.
Because this is past the 90 days permitted on VWP, and because the further flights were from/to Mexico, what evidence (if any) would be needed to cross at the land border back into the US?
I understand that because they traveled to South America they are within the rules, however the US CBP won't have evidence that they have left the US until day 96 when they try to re-enter.
This is a similar question to Can I acquire a second ESTA a month after the first ended for the purpose of transit? however that Q&A doesn't mention the land borders.