OP: For what it's worth, I think some of the extreme comments posted here display a similar lack of continence to that shown by your friend. There is also a style of sophistry on display of the same kind he will, no doubt, have exercised on his own conscience in order to justify his behaviour to himself.
You need to decide for yourself whether he is worth keeping as a friend but, whether you do or not, it is worth pointing out to him that even if he confines himself to having sex with a grown woman when he visits places like Thailand, he cannot know her personal circumstance sufficiently well to know the extent to which she is freely giving consent. There are varying degrees of coercion to which she could be subject that he may not know about. Even if she is not subject to coercion (in the sense that we would normally use the term), there is a broader cultural and economic context which compromises her ability to exercise a free choice (in the sense we normally use that expression). Because he has so many advantages compared to her, the relationship cannot really be considered as anything other than exploitative. The fact that a great many people in Thailand are very poor, the woman he having sex with is almost certainly from that class, and he is paying, all rather give the game away. Furthermore, he should reflect on the extent to which he is involving himself with a local 'sex industry' that will almost certainly has some very nasty aspects. He is unlikely to be able to judge whether or not there is a connection between his own activities and these aspects and, given that he is unable to judge, he should stay well out of it. In the best-case scenario, he is lowering his own dignity and the dignity of the person he is having sex with by paying for it.
In all probability telling him he is a rapist is likely to make him upset and angry because he will consider this to be an absurd and unjust description because, in all probability, it is. That will probably make it less likely he will examine his behaviour and see that it was wrong. It is possible he will consider such an attitude to be characteristic of an unsympathetic female response and the next time he goes to Thailand he will seek out an even more vulnerable female and will take it out on her. It is unfortunate, but that is also an aspect of how human beings work - men and women.
If he wants to be a sex tourist there are plenty of unattached first-world women who hope to include some great sex in their holiday experience. Perhaps you could help him do some research into likely destinations give him some tips on how to get laid.