You can set up Battery Alerts to receive a notification when your friend's phone's battery is low. Typically, a battery alert is sent when the battery is lower than 20%, but can vary depending on the device's internet connection and other settings.
You can turn on or edit Battery Alerts like the following:
iSharing app > Menu > Alerts Management > Battery Alerts > Turn on/off for specific friends
If your friend has an Android phone, you can receive additional notifications when their phone turns on/off. This will automatically be activated if you have chosen to receive Battery Alerts from a friend who has an Android device.