TG-6 is the go-to recommendation for new users, but you list Maldives, Red Sea and Caribbean as destinations, and those are primarily wide-angle, where TG-6 is adequate, but does not exactly shine.
LX10 and LX100 are more popular among video shooters rather than photographers - I'm not 100% certain about LX100, but LX10 has vignetting issues with wet wide lenses, which go away with the cropping that the camera automatically imposes while shooting video.
E-PL10 is a decent entry-level ILC, with a good budget housing available for it from AOI, but it's hampered by contrast-detect autofocus. It shouldn't be much of an issue in reef scenes, but fast-moving pelagics, macro, blackwater, etc, can prove challenging.
If I were building a new setup right now, I'd probably go for a Sony RX100VA or a Sony RX100VII. The latter has considerably better macro capabilities, but at the expense of interchangeable port complexity and higher price. With a higher budget, I'd look at Olympus E-M5 III (same sensor and AF as E-M1, but at lower cost and smaller size), or Sony A6400/A6600 (better AF and sensor, but not as good a selection of lenses specifically for underwater photography as the M43 platform offers). I wish Sony ZV-1 had more housings than just the one from Ikelite - it's the equivalent of RX100VA, except that it has a hot shoe, which removes the constraints of the slow-recycling pop-up flash for triggering strobes.
As for instructors, I would recommend @alextyrrell (http://dive4photos.com/), but he's based on Koh Tao in Thailand, which is outside your friend's normal travel route. Otherwise, liveaboard photography workshops are quite useful, as you not only receive lectures, but also observe a number of other photographers in action.