Video recording using external microphone and smartphone
If the microphone is hooked in to the phone, then it should be an input to the camera app and it should record both at the same time.  If the microphone was not linked to the camera app for some reason but was recording separately, it isn't that hard to sync, just clap on camera and look for the spike on the waveform and align it with the clap.  I wouldn't expect it to be a problem though.