TM—Thanks for asking this question. You only have one chance to make a first impression.
If a company has a casual Friday and that’s the day of your interview, you should dress up as you would if your interview was on any other day. Companies will relax their dress code for one day because it’s a morale booster for employees—but that rule does not apply to job seekers. Once you’re hired, you can be a part of ‘Casual Friday.’ So you are correct and I think you won that bet!
I respectfully say: Get up, dress up, and show up.