I have no experience with the UEF nib or EF (of any maker tbh!), but I have read people's opinions/reviews, and sometimes a Sailor EF does turn out finer than a UEF.
The Japanese makers are very precise, but even with that precision and good quality control, you still get a margin, so any nib size can turn out broader/finer than intended.
The nib nook is a great place to start, also use google images to compare nib sizes, that's what I used to do when I wanted a new nib size (still do occasionally!), but even these comparisons aren't always super helpful.
Eg when I bought a Platinum 3776 Century with an M nib, I was surprised how much finer it was compared to my other Ms (even compared to Sailor & Pilot), so I thought the B nib would be great, also because everyone kept saying japanese = one smaller than Western and the Sailor B I tried back in the day wrote finer than my Pelikan M at the time... Looked at comparisons on google, instagram and other places, the B seemed good, ordered it and good God did that turn out to be a paintbrush!
I like the B nib, but for my taste it's a bit too broad so doesn't get much use....
Don't get too hung up over comparisons (though definitely invest time in them!), also always apply the golden rule of gift giving: suss out the victim subtly to see where to go. Eg get him into a talk about pens and how he likes the nib size you gave him prior and how useful it is and whether he's interested in a slightly broader nib.
Getting a gift-victim into conversation is always great and you'll be even smugger when the intended is super surprised about the gift later (at least I am!).