There are 3 flavors of 5G. There's the ultrawide band, which is the one everyone promotes as being stupid fast but with a signal that has trouble penetrating a piece of paper. There's a mid-band that has a slightly longer wavelength but speeds somewhere between UWB and 4G LTE. And then there's the low band, which is basically just 4G LTE with a new name. What is in use depends on the carrier and location, but the best reception would be the low band, which is one of the most popular versions. It's supposed to be on par with 4G LTE, but a lot of people do report slower speeds on it. Signal strength should be about the same.
I assumed the 5g and 4g were always on the same tower.
I believe this to be correct. The 5G networks are being built on top of the existing 4G LTE networks, not replacing them. 5G is still too new and sparse to even begin thinking about taking any 4G networks offline. Heck, they just recently started taking 3G/CDMA networks offline. So no need to worry about having a 4G phone and suddenly losing connection if entering an area with 5G.
So were you right in your assumption that she wasted her money on a 5G phone? That depends. All things being equal, 5G doesn't have better signal strengths as 4G. At best, it'll be roughly the same. The one thing we don't know is what the old and new phones are, and it's possible the new phone simply has a better radio and internal antenna design that helps it pull in the signals better than the old one. Or it could have a worse design. Not every phone is created equal in its reception abilities. Plus she likely has a better phone now in other ways that make it worth the price for her even without a better signal (storage, performance, screen, etc).