I have a friend who lives in Thailand and his wife owns the property the land and the business - she has been sick for the last year. She has concealed the fact that she has had HIV for 10 years from him - he has not contracted HIV. What happens now?

Does the woman have a last will and testament? If not the estate will be divided among the statutory heirs, which are grouped into six classes.

Read this: http://www.thailawonline.com/en/family/inheritance-and-wills.html

The link above includes a very important statement:-

Foreigners are normally not allowed to own land in Thailand. However, a foreigner who acquires land by inheritance AS A STATUTORY HEIR (Clause 93 of the Land Code), the ministry shall permit inheritance of the land to an alien. However, it’s rarely done and the alien will have a delay of not less than 180 days or no more than one year to transfer the land to a Thai national. (See clauses 93 to 96 of the Land Code). If a foreigner acquires Land by inheritance as a legatee (through a Will) than the administrator of the estate will dispose of the land and give the proceeds to the foreigners.

As a foreigner one cannot inherit land via a will !!