"Can I use a friend's home address to maintain my residency?"
Your residence for tax purposes is not determined by your mailing address. If it were, everyone would be renting a PO Box in a no-income tax state!
Here is Illinois' definition of resident: https://tax.illinois.gov/questionsandanswers/answer.369.html
The general rule is that your domicile does not change until you abandon it in one state and re-establish it in another state (or country).
The address you enter on your tax return should be the address you want the IRS or state taxing agency to use in case they have to send you a letter or a check.
Also: rental income from a property located in Illinois is taxable by Illinois regardless of whether you are an Illinois resident or not.
Bottom line: if you have not yet established a new domicile outside Illinois, then you should file an Illinois resident tax return. And it's OK to use your friend's mailing address if that's where you want your tax-related mail to go.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
