If she is on a "four-month backpacking trip around North America" and tried to return to the US, she has exceeded the 90-day limit allowed by the Visa Waiver Program (which counts days both in the US and "adjacent territories") and is now an illegal overstayer. The unpaid labor stuff and getting refused entry to Canada is icing on the cake.
For the record, I'm no fan of ICE/CBP, but it looks like they're just enforcing the law here.
For the record, I'm no fan of ICE/CBP, but it looks like they're just enforcing the law here.