Think any one of them may be more suitable for new divers? Red Sail/Tortuga looks like a pretty big operation on Grand Cayman...
Red Sail is a big operation, but they use big boats for their operation on 7MB/West Bay so they are not cattle boats, and their Tortuga Club dive op on the East End is a slightly smaller op; we have dived with them (both East and West) and didn't have any complaints.
A lot of people really love Ocean Frontiers and I think that they are the most highly regarded, popular dive op on the East End - we have also dived with them and found them to be excellent. Sometimes they are booked up and that is why we initially decided to give Tortuga a try.
I think he would be fine with either group, and Red Sail Tortuga is a little closer to Rum Point so it might be an easier drive, but I believe that Tortuga only offers 2-tank dives in the morning and 1-tank dives in the afternoon. While Ocean Frontiers offers 2-tanks both in the morning and afternoon, and the afternoon option might be a little easier for your friend since he will have to drive to the East End.
Check them both out to decide what sounds best, but he shouldn't delay too long because they might book up, especially the highly popular Ocean Frontiers, as
@Damselfish has advised.
White Sand is a smaller op (6 or 8 divers?) so that might be desirable for a new diver - but I haven't dived with them yet so cannot give an opinion.
In general, most Grand Cayman dive ops are just fine, there is too much competition for a bad op to last very long.