Neoprod
Honorary Master
If he goes for A320 I would suggest getting one with 4x DIMM slots like this https://www.wootware.co.za/asus-prime-a320m-a-amd-a320-am4-m-atx-desktop-motherboard.html
I did a budget build for my brother recently and picked the Asus board because it supported RAM speeds up to 3200Mhz...most A320 boards are capped at 2400/2667. It does only have 2 dimm slots but that's not going to be a problem for a long while if you go 16Gb from the start.
Going B450 avoids thinking about stuff like that which is kind of nice but I wouldn't buy a cheap B450 just to have a B450. The cheap ones are bad products (even if it wouldn't matter with an APU, we shouldn't buy them).
If someone finds a way to shoehorn a Tomahawk into the budget, that would be great.
....you can pick those out yourself with the wife (you should do the same for the case as well - that kind of stuff is like personal preference).