there is such a thing as shopping addiction. also i believe anyone can develop an addiction to anything if done in excess and is out of control. you're a good friend to be concerned.
as well, there is some suggestion that some, certainly not all, people who have drug or alcohol addiction may transfer their addiction to something else while in recovery. i don't know, nor need to know, if your friend is alcoholic or a drug abuser or in recovery .what i do know tho is i have seen friends of mine in recovery replace their drug/alcohol addiction with some other self destructive addiction, just me. this does not have to happen but there are times i have witnessed it first hand.
sorry i couldn't be of more help.