As a US citizen you DO NOT have to obey them in the same manner as a police officer and their rights to detain you are limited. Unless they were placing her under arrest she was free to leave and drive on. Most ICE agents lack the same training that your average state police officer would be forced to go through and thus are far more likely to do something stupid.
A federal court agreed, ruling that agents can't rely on factors such as race, speaking Spanish, wearing workman-like clothes, and location [...] to meet the standard of "individualized suspicion."
But in September, the Supreme Court paused that previous ruling, saying immigration agents can use those factors as reasonable suspicion to stop someone. (Legal proceedings continue on this case, however.)