two of my separate clients flew out of there yesterday and both said it was crazy busy, i'd say it would be worth paying for the fast track based on that!
yes
i'm from there agreed cavtat is lovely and you can reach dubrovnik via boat. if you still want to be somewhere closer, definitely stay in the lapad area rather than the old town. the old town is busy, noisy and expensive. charming of course but lapad is close enough so no need to stay right in the centre. i'd recommend doing the city walls and a big tip is that the ticket to the city walls includes a ticket to the st lawrence fortress as well, and you can visit the 2 parts within 3 days, it doesn't have to be on the same day. next to that fort is dodo beach bar, highly recommended for cocktails!
locals' favourite restaurants would be pantarul for traditional food with a modern twist, trinity/shizuku for asian fusion (amaaazing) and kamenice for local seafood. don't get the local taxis as they're really expensive, use uber because it's very fast (less than 5 minute wait wherever you are) and much cheaper. also you can feel 100% safe, in taxis or anywhere really, we have literally 0% crime here. it's also very clean! and not too busy just yet.
my top suggestion would going to lokrum island, 10 min boat ride from the old town, there are peacocks there and it's incredibly beautiful
keep in mind that it's very warm here right now, it was 30+ degrees celsius today so definitely bring swimwear and go to a beach or do a boat trip. most houses have air conditioning so they should be fine in their accommodation but it's very warm outside for sure. lots of sunblock and spf, and aloe vera for after.
i do hope this is useful - sorry for the rant, i'm a tour guide so i kind of get passionate about this
if you have any specific questions let me know and i wish a lovely holiday to your friend!