On a rough estimate, a billion THB is around 23 million GBP. Even THB 100,000/month and buying a few Audis, BMW's, Merc's, Louis Vuitton handbags, and paying for a THB300,000 operation wouldn't put much of a dent in that.
She must have been siphoning off big time. I wonder how she tricked him into selling his property.