If he needs a keyboard, mouse and monitor, you need to tell him to ah heck off. Otherwise, use one of these sites to pick the pieces:
http://www.logicalincrements.com/
https://pcbuildsonabudget.com/best-gaming-pc-build-under-500-dollars-2018
https://pcmasterracebuilds.com/
https://pcpartpicker.com/
Keep in mind that these are basic, entry level specs when you're building in the $5-600 range. When I'm building a basic gaming platform, I start with the Ryzen 3 and 8GB of RAM. Any GPU with 4GB RAM is the starting point. Try to fit an SSD into the unit, even if it's only a 128GB piece. Please use at least a Bronze, 450w PSU: you won't regret it. Shop the sales at Amazon.com and Newegg.com using https://slickdeals.net/deals/computer-parts/ A great way to save money is to use the cheapest, on-sale mATX case you can find. I've built systems using Rosewill mATX cases for as low as $9.95 (US). They work fine, and like anything else, you can upgrade later.
If you're trying to make a profit building an entry-level gaming computer, pretty much forget it. If you're doing this as a favor for a friend, get the money up front, and make sure he appreciates what you're doing for him. And don't forget, Windows 10 costs $100, something no one ever mentions until the very end...