She was an obviously-distressed, unarmed woman.
Why would you rule out altruism? Soldiers--especially soldiers from Western democracies like the USA--routinely render aid to civilians out of the goodness of their own hearts.
She was at the landing zone.
They were called to a specific location and she was there. She was unarmed and afraid. She spoke English well enough to communicate that she had been with Dutch and his men and knew what became of them. Further she could describe the threat they were up against. They absolutely would want to apprehend and question this woman so that they could gain some insight into the fate of their team.
The real question is why was she still on the helicopter the next day.
The fact that she is on the helo the next morning suggests that they returned to base and came back, bringing her with them, which would be unlikely for numerous reasons. First, she's a valuable asset that needs to be interrogated and protected, not brought back into danger. Second, she has no skills necessary for the extraction, she's just extra weight. Third, they likely would have ascertained by then that she had been with the rebels so they would not want to give her the chance to cause trouble for them.
The alternative is that the helicopter never returned to base during the entire night where Dutch fought the Predator. This would have exceeded the helicopter's fuel capacity, or required them to land in the jungle and spend the night there. Having her in the helicopter the next morning is a gaffe.