Sounds like you've already got the info you need but I thought I'd throw in my two cents...Overweight doesn't necessarily have anything to do with being able to scuba dive or with having to wear a wetsuit as you've read from some of the responses. So, if that is the only thing holding her back, I say she should go for it! If there are medical conditions or just a plain old reluctance to dive, that is another story. But, I've met lots of divers who were extremely reluctant to go but were trying to appease their spouse/relative/friend and ended up falling in love with scuba diving after a great first experience. I was one of those (trust me, I was very reluctant to go and almost quit in the first 5 minutes of the dive) and now I teach scuba! This is probably why so many people try to talk their friends into diving...b/c they just know that they'll end up loving it, too. However...as has been pointed out on this board many times - scuba is not for everyone and there are some people who just shouldn't be in the water.
Back to the weight issue...our shop stocks suits up to 6XL and the largest diver I've ever gone diving with was 6'4 and around 450lbs. He fit quite comfortably into a 2XL bcd, was certified and only needed 20lbs of weight w/a 3mm shorty. A fantastic diver and all-around great guy and an example that we shouldn't always judge a book by its cover.
IMO, if you have time, you should totally get certified first! It will make your diving in Phuket so much more enjoyable because you will have hopefully learned the skills to take care of yourself and to feel comfortable in an underwater setting. Plus, getting certified ahead of time will open up WAY more diving opportunities there...deeper dives, night diving, some of the more intricate diving, etc. And, if you are certified, you can also rent an underwater digital camera and take some amazing pictures of your dive trip to post back here on ScubaBoard! As a first-time or Discover Scuba Diver, taking pictures can be a difficult proposition and I generally don't recommend it but if you are certified and comfortable in the water, then you'll have the ability to capture these memories forever...and share them with your friend and maybe get her motivated to dive if she hasn't already.
Good luck to you and your friend!