Give me tips I can give him, decent food places, where to buy, where to go, how to not fall into traps, good food that is cheap to buy and make there, etc.
Rosszlanyok.hu/beszamolok.hu for his whoremongering needs. Simon's Burger is a cheap Five Guys knockoff and it gives away free food in exchange for positive reviews, the burger is literally so bad it's inedible though, avoid it like the absolute plague unless you want to shit your guts out, I'm serious. If he wants to try good (although in a bit fancier of a presentation than usual) Hungarian food do 'Retek bisztró'. Never go into any restaurant that has hostesses in front. If he misses home, wants to splurge a bit and enjoys fine dining try Essencia, it's quite good and I'm fairly sure the owner and chef Tiago is Brazilian.
Never EVER use the yellow/blue "Euro" ATMs, they are scams.
Never change money on the streets. In fact, you don't have to at all, you can pay by card in 99.99% of places, it's mandated by law. Occasionally they try and lie about the machine being broken to foreigners but you have to be pushy in those cases. Get Wise or Revolut and you're golden, they have the best exchange rates. 'Skrill' is mega based as it has precedent of telling the Hungarian IRS to go fuck themselves and not giving out any info at all, but it's more of a substitute for Paypal than it is for a regular bank account. Don't open an OTP account if you make a bank account, just choose anything else, it's the gayest fucking bank there is.
You also have to be insanely fucking racist, if someone speaks Hungarian and looks Indian, they are gipsies. Don't talk to them in public, don't show the time on your phone when they ask for it, don't give them money, don't start a gay little google translate back and forth with them, just walk with a purpose and ignore it if they try and flag you off in the subway or something. If you do this you have a decent chance of avoiding getting mugged.
If you're going to be taking Taxis, NEVER use anything that isn't Bolt or Főtaxi (the two largest providers) or it'll be a very expensive ride for you, you'll get ripped off, I think Bolt is the best because the app does the calculations and the driver literally has no way of ripping you off.
Also betting that he won't learn Hungarian for shit, I can't fault him for it, but he has to temper his expectations.
Budapest is irrationally expensive, just be prepared for it mentally.
Loads of expat places (Hoff House being one of the better ones), I'd recommend going to them so he won't feel as lonely, Budapest is a bit alienating to foreigners.