In your example you are stating that they should use 90% of their credit.

https://www.equifax.com/personal/education/credit/score/credit-myths-facts/

1. Approaching your credit limit will not negatively impact your credit scores.

FALSE. Even if you pay off your credit cards every month, if your credit utilization ratio is high, it may impact your credit scores. Your credit utilization ratio represents how much revolving credit you're using compared to the total amount available to you. Revolving accounts, such as credit cards or personal lines of credit, do not have a fixed number of payments. Installment loans, such as vehicle loans, do. When you pay your vehicle loan in full, the account would be closed and marked as paid. Keep in mind there are many different credit scoring models with different ways of calculating credit scores.