HTC One (M8) — Will updating my HTC M8 tru the recovery mode (update tru SD card) fix the bootloop error?

My brother was updating his M8, ( a couple of weeks ago, so I asume it was the marshmellow update). In mid update the Phone ran out of battery and and now the phone is on a bootloop. The thing is that he cant loose his data. So I was wandering if I downloaded the HTC M8 Marshmellow update into the SD card and do the update it from the recovery mode it might fix the bootloop.

Im asking to see if it will work and if it will save all the phone data.

Thanks in advance