I've been diving with 2 of the above - Ocean Explorers and Dive Adventures. I prefer OE. Full disclosure, haven't been diving with OE since original owner sold, that said, I've had other friends down and hear the same quality and high standards set have been maintained. Small op, smaller boats, not catering to the cruise ship crowd.
With the exception of Octopus, the other 3 are on Dutch side and probably do many of the same sites. The HMS Proselyte is the 'named' dive site, 1770 ship, wrecked on Man o War shoals in 1801. Nothing left but big encrusted anchors, dive about 50-65 feet. Many of the closer sites are patch coral, small reef kind of stuff, couple boats/ships - RORO (roll on/roll off), tug, sail. Most dives in the 50-75 foot range. There are supposed to be deeper/better/diverse sites further off shore, I've never been out to them.
Pleasant diving, not as rich or diverse as some places, but you never know what will cruise by... Happy to check my log books for specific sites/depths/sightings etc if your friend wants more details.


(OE sign and Frenchmans Reef)