Ask to use the phone. He doesn’t have to say anything or surrender the phone. On what grounds did they have to detain him over his phone?
And then they say no. One of the fun parts about border checks is that technically speaking they aren’t detaining you. Because they’re the gatekeepers on whether or not you’re allowed in the country they have the ability to not let you in until you meet their requirements without ever actually arresting you, and while you have the technical ability to arrange transportation out of the country that can be rather difficult when you’re in a room with no phone or computer. “We have suspicions and we’d like to see...” is a very powerful tool when placed at border checkpoints.