Chargrilled

Have you tried re-seating the CPU?

Is the card just for gaming? What resolution are you currently playing at/wanting to play at? Do you have a monitor with FreeSync, GSync, or neither? Are you going to be playing the newest, most demanding games, or not? What budget are you working with?

1050 Ti isn't a bad idea. The 2400 wouldn't exactly bottleneck it so it might give performance a moderate boost. However the 2400 is going to be showing it's age soon enough. The RAM may be letting you down too seeing as it's DDR3 in single channel at a low clock. A full upgrade would be best but of course that's not an option, so a 1050 Ti could be enough to hold your brother over until he can upgrade the rest to match.

Changing the BIOS is perfectly safe if you only touch what you mean to. You can search for XMP so that it will only show you the XMP settings. XMP settings are a drop-down box with usually only 2 options. When you Save & Reset it will even list all the changes you've made before it resets so that you can review them and make sure you didn't accidentally click something. But it's up to you I guess. In any case, @Xplo1t's method seems like the best for testing the slots.

Restart while the RAM is in the incorrect position, get into BIOS and set them to XMP I or XMP II, then power down, re-seat in the correct position and try to boot. Unsure if XMP settings will carry over after re-seating but it's worth a shot.

Put the RAM back in the correct config and on boot hit F2 or Delete to get into your BIOS. Search for XMP if it's not on the first screen you see. Make sure it's set to XMP I or XMP II, then Save & Reset. See if that works.

If you have a GPU and it's not outputting, try using the iGPU by swapping the cable from the GPU port to the same port on the motherboard rear IO and see if that will boot (with the RAM in the right slots)

So you have them in A1, B1, C1 and D1 according to this diagram? Try the things @suchamoneypit mentioned above.

As long as all the DIMMs are working properly you should be fine just powering down and re-seating in the recommended slots. Edit: As with any tinkering, of course make sure you turn off power at the wall and wear an ESD strap (if you don't have one, touch a faucet or your case before poking any expensive components)