You can't. The guy's wife has a serious disconnect from reality and is using shopping as escapism. Unless you're a therapist or a financial counselor, your help won't be enough and they'll see you as a soft touch and expect you to give them help whenever they go into the red if you give them actual money even one time.
If the situation is dire, then he needs to get counseling - both financial and marital - since his wife is a huge problem.
He can ask for help at his church, see about low or no cost counseling and go to a food bank if necessary. He should look into closing or freezing their credit cards, and shutting off the cell service (or going to a low cost phone only thing - hell even Ting you can lock out things like data usage or text usage so it's just a phone).
If it was me in this situation, I'd likely be getting a divorce. So sorry your friend is dealing with this.
But you can't and should not be inserting yourself into this.
It really, really is way out of your (and most average friends) league to deal with this type of situation since it concerns both their marriage and money and communication between the spouses. Be there if he needs to talk and encourage him to find professional help, but don't get in the middle of that mess - because it may harm your relationship in the end.