Believe me, it's the cataracts. I had the surgery last year; you should see a big improvement afterward, though you may still need glasses.
The font in Rick's guidebooks is not small. I have the 2020 edition of the London book (634 pages). It uses 10-pt type for the basic text; the italics used for some inserted directions within the walking tours are 9.5-pt type, as are the sans-serif panels used for side commentary. Maps are different and may be more challenging. There are other companies' guidebooks that are definitely harder to read for those with imperfect vision--one I tried to use had large chunks of 8-pt type, or perhaps even 7.5-pt. (I even complained to my optometrist about my inability to read such guidebooks, pre-surgery.)
I must warn you that if you cannot read Rick's guidebooks, you are probably going to struggle with at least some of the explanatory material posted in museums. My cataracts worsened during my lengthy 2019 trip, and reading museum info became really difficult for me. I also had a hard time reading my tablet computer.
If I were you, I'd check on what one of the discount eyeglass places would charge for an exam and a pair of glasses. It might make a lot of difference to your enjoyment of the trip. On the other hand, new glasses may not help much (or perhaps not at all), because they won't fix the cataracts themselves. I struggled to get workable glasses while my cataracts were forming. Take something you can't read easily (or at all) with you to the eye exam. I would hope you'd be able to tell from the exam (as the optometrist swaps potential lenses) whether the proposed lenses would make much difference, so you could opt not to follow through on purchase of the glasses if it appears they won't really help.
One other thing: Until you have the surgery, you need to be sure you have really good (excessive) lighting when you try to read. That will probably help a bit. I resorted to using the flashlight on my smartphone in museums before I had the surgery. (I still use it today in museums with extremely low light levels.) It is possible to buy an illuminated magnifying glass. However, I find using any magnifying glass frustrating and would be too impatient to use it for reading a guidebook.
I think your young friend is due for an eye check-up. Seriously. I am 71 years old and am not 20/20 even after the surgery and with brand-new glasses. If I can read Rick's guidebook, she should be able to with the right glasses.