hi
Yes it will be very hot at that time. I would advise that they be off shore, swimming, or resting in their air conditioned room during the hottest times of the day. To be very honest I wouldn't go there at all at that time of year, I think it would be too hot. We were in Croatia in May a couple of years ago and it was too hot for me on the mainland during the day.
We spent a couple of days in Dubrovnik looking around, then we travelled up the coast to one of the bigger islands off Split (can't remember name) via the car ferry but I'm sure there are more stylish ways of doing it, it was comfortable but basic. I wish now we'd stopped in Korcula, lovely looking island. They could also just day trip out to various islands near Dubrovnik. However the car ferry was a great way to get to Split which I thought was a fantastic city, but wouldn't be everyone's cup of tea.
We got the bus back to Dubrovnik, spent a few nights in Makarska which is kind of like a holiday resort, perfectly pleasant for a bit of swimming.
We really liked Croatia, I would go back in flash. Particularly liked the people they were very no nonsense, wouldn't suit holiday makers who want to be treated like royalty.