I've never dived Koh Chang, but I know a guy who worked there as an instructor for a while and he told me that the visibility there was usually fairy poor (maybe 12 meters on a good day). I've dived a lot of places where the viz was poor, and often greatly enjoyed the dives nevertheless. I know it's almost a cliche, but it's true that "what is important is not how far I can see, but what I can see" and if there's plenty of interesting marine life or a good wreck within 6 or 8 meters of me, I'm happy enough. Of course, if the viz is only 10 meters and that whale shark is 12 meters away, you'll never know it's there ... Viz at Koh Tao-area dive sites varies a lot, but is almost always better than 12 meters -- we had about 25 meters or more at White Rock and Twins yesterday, and 35 meters at Sail Rock today.
As for your girlfriend's course: A number of instructors in Phuket, here on Koh Samui, and elsewhere in Thailand like to bash the quality of open water diver courses on Koh Tao. From what I've seen, some of that criticism is justified - for some Koh Tao instructors, not as a generalization applying to every instructor on the island. Some of the comments that I've heard aren't really pertinent to whether the instruction is good: for example, an instructor smokes ganja with his students after the course is completed -- maybe that's not professional, but how does that relate to the quality of instruction before they get high? It doesn't. The most common criticism is that there is usually a high student-to-instructor ratio in Koh Tao OWD courses, so there's not much personal attention paid to each student and it's common for instructors rush their big groups of students through the skills exercises before the students have really mastered the skills. It should be easy enough to get around that problem: your girlfriend should insist, before paying for the full price of the course, upon a low student/instructor ratio (or even one-on-one instruction) and insist that she be given ample time to master the skills without being hurried. With all the competition on Koh Tao (something like 40 dive centers), she should be able to find someone willing to accommodate her.
