There’s no practical way to directly sync a phone’s flash to your NEX-5R like a normal camera flash. Smartphone flashes aren’t designed to integrate with a camera’s metering or trigger timing, so using the phone as a true synchronized flash isn’t really a workable solution.

A possible workaround is to use the phone as a continuous light source instead of a flash. On phones with dual-LED “true tone” flash, starting video recording with the flash enabled may keep the light on, letting you aim it at the subject while taking the photo. Torch mode can also work as a constant light source.

This won’t behave like a camera flash, though: exposure and metering won’t be automatic, and the light output is limited. If your subject is stationary, a better option may simply be a longer exposure rather than trying to use the phone as a flash.