She can travel to South Korea after the treatment for at least 4 weeks.
In this case, direct flight from Bangkok to Seoul is about 5-6 hrs. And since she has history of previous VTE, she will have risk to develop VTE again. I would recommend her to;
1) take anticoagulant regularly as prescribed – if she is taking warfarin at the time of traveling, then continues the medication as usual
2) wear loose and comfortable clothes
3) do calf exercises at least every half hour – raise your heels, keeping your toes on the floor, then bring them down 10 times, then raise and lower your toes 10 times
4) choose aisle seat and walk around whenever you can
5) use compression stockings
And also advice her to recognize signs and symptoms of VTE.
In case if she is planning for a long distance flight (>8hr) and already free from anticoagulants, I would recommend her to use LMWHs such as enoxaparin SC for prophylaxis for VTE during the long-haul flight.