You don't have to be in debt, but you do need open credit lines. Having debt on them actually makes your score worse.

Her score likely went down because she closed out a credit line, i.e the open loan, so technically the "i have an open 5yr loan ive been paying on diligently" is no longer part of her score. The fact that she did pay it off is part of that score, but its weighted differently.

If she instead had 40k of credit cards she had open for 5yrs, with zero debt on them, her score would have gone up. Just having the account open, even not using them, shows a high "credit to debt usage" ratio and "a long time open loan." Both of those make up about 45% of your "credit score."

So no, you dont have to use a CC every month to keep a high credit score. If you want a high score, you want to open a credit card or 2 for their max value until you get about 30k-40k of total credit, and then don't use them at all. Not a bit. Never close them. The "long time accounts" + "high amount of debt not in use" + "never delinquent" is roughly 80% of your score. You can sail into the 700s/800s if you dont have any other credit hit.