It is difficult to understand but it is very hard to leave your country and go thousands of miles to another country with worse weather and a very different culture and far from her family and all who are dear to her. Life at times must have been very difficult for her.
Going back to her home for a long visit must have been very unsettling and if her family missed her, had never met their grand daughters and wanted her home I can see that the comfort of being with familiar family whom she hasn't seen for many years must have been tempting; food, weather, close family and so many memories may have been so unsettling - then the old boy friend appears, by accident or design.
I hope that when she gets back she finds life back home has all the compensations she is looking for to justify walking out on husband and family. I suspect that when she gets back there living as a Thai citizen in her own country, she will find it is not as easy as she thought and she finds she no longer fits in, once the nostalgia for lost friends and romance has the bloom rubbed off by the banalities of everyday living she may well regret her decision. Remember all those rekindled romances on Friends Reunited that ended in tears?
I do not understand how she can walk out on her children they way she is doing so, but I can understand how a long visit home after, perhaps, a long absence, will have deeply unsettled her and led her to the decision she has made. I suspect it may not be all she is hoping it will be.
In vastly dissimilar circumstances a friend made a similar decision; to return to somewhere she had been happy. It was a disaster. The past is a foreign country: they do things differently there.