This app was mentioned in 9 comments, with an average of 1.78 upvotes
What do you use right now? Only a smartphone or tablet? If it's Android I see an app that you can use with the Arduino IDE: https://play.google.com/store/apps/details?id=name.antonsmirnov.android.arduinodroid2&hl=en
But long term you really ought to get a PC, doesn't matter whether it is Linux/Mac/Windows.
ArduinoDroid should work if the tablet has USB OTG (host) support.
I'm not sure what "simple stuff but isn't much into tech and he deosn't want to walk around with a laptop" means. He can also sit down with a laptop, or he could even buy a desktop. He knows that he needs to write programs ("code") to make use of an Arduino? I think I wouldn't be happy to program on a touchscreen keyboard. Of course he could use a Bluetooth keyboard, but then that's not so far away from a laptop anymore.
What does he want to do? Did he already have a look at https://www.arduino.cc/?
I assume you mean for a phone or tablet? The closest to Arduino IDE on android is probably Arduinodroid. It's mostly free I think with pay-for extras, but has limited boards - a few arduinos and digispark boards.
No idea for Apple, sorry
I didn't look into it but maybe this has support for it?
Android Arduino IDE: https://play.google.com/store/apps/details?id=name.antonsmirnov.android.arduinodroid2
Maybe this app works? Never tried it.
https://play.google.com/store/apps/details?id=name.antonsmirnov.android.arduinodroid2
One up on ide in crostini.
Also, can use android apps like ArduinoDroid https://play.google.com/store/apps/details?id=name.antonsmirnov.android.arduinodroid2