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I'm very sorry this happened. Unfortunately you can't undo it. It is almost impossible to recover overwritten data from the iPhone.
Your only hope is iCloud backup or iTunes backup of his iPhone. First connect the iPhone to the computer and open iTunes. Select the iPhone. Click Restore Backup. From the list of backup do you see his backup point ? If so, restore to that backup. If you don't see his backup point, then next is iCloud backup.
If there is iCloud backup of his phone available, then you can restore the phone to that backup. On his phone, go to Settings > iCloud > Backup and do you see a more recent backup there ?
If so, you can erase that iPhone and restore to that iCloud backup. Go to Settings > General > Reset and tap Erase All Contents and Settings. After erasing the iPhone, it will ask you to setup it. While setting up, select Restore from iCloud and this will restore the iPhone to his most recent backup.
If both are not available, I'm sorry the old information is lost unfortunately. You have to erase the device and setup it as new to get your data off it. You can do the erase from Settings > General > Reset > Erase All Contents and Settings
If you have any questions or would like any additional information, please don't hesitate to ask.