I don't think there's any evidence of mental illness. I think we should also keep in mind that the Chinese have a very different culture from ours, and also every marriage is different. Maybe there was some love, but also convenience? Maybe they are both just independent people.
I agree with you about their marriage. My focus is on her travel in Mexico. The connection between risk-taking and bipolar disorder is well-documented, so it doesn't seem unreasonable for the thought to cross one's mind.

"Compared with those in an individualistic society," say Hsee and Weber, "people in a collectivist society may be more likely to receive help if they are in need. As a result, the adverse outcome of a risky option may not seem as severe to them.
https://www.chicagobooth.edu/capideas/sum98/hsee.htm