I'll put my banker hat on for a bit. There's a bunch of car payment calculator programs out there on the 'net that will work out your monthly payment given the price of the car/loan amount, interest rate, and down payment.
Punch in the numbers for $0 down or $5000 down at whatever the interest rate is and it'll spit out your monthly payment.
https://www.carpaymentcalculator.net/
Now, here's the fun part. Your buddy could be right in many cases since the dealership will screw around with the terms & rates on the loan to max out their profits. They have a nice table of down payments & credit scores that they'll use to adjust the interest rates on their loans and fuck you over. Let's you should get a straight up $90/month difference between $0 down and say, $4000 down at the same interest rate. What the dealership will do is jack up the interest rates a few points so that you're only saving $25/month, they won't show this to you but you can work it out by adding up all the payments for the term of the loan then working it backwards through the calculator.
You can have the payment calculator ready on your phone when the dealer gives you the numbers for their loans. If they're screwing you for $10 a month it ain't too bad and you can probably live with it. $100/month and you can tell them to stick it where the sun don't shine.