Overwatering or underwatering or other cause?

Plants this size/age are going to need more attention than watering once a week. If you were dealing with normal houseplants, this wouldn't be a problem (even if they needed watering more often) but cannabis is a more finicky thing. So I think this dried out - which wouldn't have been a problem if it hadn't STAYED dried out for several days. I lift every pot every day to see if the plant in that pot needs some attention and, if so, give it immediately. The only other thing I can think of is, since your friend gave it so much water (and was it PH'd properly?), did it really need you to water it again? Was the pot super light? Did you perhaps overwater? Whatever the cause, you need to deal with what IS... keep the plant in the shade if you put her outside (you don't need to add heat stress to your problems) and water/feed only when the pot is lightweight - and cross your fingers. She'll never be a gorgeous plant again since it looks like she's coming to the end of her life but at least you can preserve the harvest you can get from her. Good luck!