There seems to be a great amount of confusion over this issue. The fact that the person in question has a 10-year visa (which I would guess is either a B-2 or a B-1/B-2) does not automatically establish eligibility to enter the US for any particular trip during that 10 years. For example the B-2 allows for entry for the purpose of tourism. If she showed up at the border and said "I'm planning to marry my boyfriend in Florida" then this would not be compatible with a B-2 visa, and she'd be denied entry. Especially since the visa could have been issued up to 10 years prior, there's absolutely no way the US Embassy in Thailand could possibly determine if a potential future trip some 10 years later would be for the purpose of getting married, no matter how much documentation was presented about the trip for which the visa was issued. Similarly, if someone planned to come to the US for several months, it could not be determined up to 10 years in advance if they'll have enough money to support themselves, even if they were independently wealthy 8 years ago. Times change.

So, having the visa gets you over a lot of hurdles, but there are still a lot of things that simply can't be known in advance of arrival. I guess one alternative would be for the US to switch to a system similar to China, where you basically can't get a visa that's valid for a long period of time, allowing the embassy staff to verify your bona fides shortly in advance of each visit, but I don't think that would go over very well.