1 TB SSDs are 35-60 dollars.
1 TB HDDs are 22-50 dollars.
2 TB HDDs are 40-65 dollars.
2 TB SDDs are 60-90 dollars.
Clearly, price shouldn't be an issue because one of these drives that give you 10 times the storage is the cost of 1 new release, and the theoretical person who just bought BG3 and Starfield just spent 120 dollars minimum. So theoretical person let's do some math!
Seems really silly to complain that you ran out of space on your PC. Get another drive. If you've filled up your SATA ports, get a PCIe SATA card. If you have all your onboard SATA slots full, plus your PCIe slots are full, plus you've upgraded all the drives you could to at least 1 TB, that typically gives you at least 2-4 TB total. BG3 is taking up 150 GB that you reserved for gaming. Uninstall it if you want to play Starfield. If you don't want to play Starfield that badly then you have your answer.
Clearly, the real answer is that this person needs another drive in their computer. They act like the OS drive is the only thing that could possibly exist in a computer. Worst case, go get a USB 3 drive and toss Starfield on that.