I have a friend being sued by a collection agency and they want me to
defend them because of my knowledge of the Fair Debt collection
I am not a lawyer in IL, but representing someone in Court is pretty much limited to lawyers. It is unlikely that the judge would even let you sit next to your friend at the defendants table or cross the bar.
Yes. If you are not a licensed Illinois attorney, you cannot represent anyone in Court. Doing so would be unauthorized practice of law, which is a crime.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.