This is very misleading! First, 35% is not a strong correlation between stress and having an upset stomach. Moreover, (and this is a general rule of thumb for life) correlation does not imply causation! Even if we found 99% of the population was stressed when they had an upset stomach, this does not imply that this caused the digestive problems.
After all, couldn't it be that being in a mental health facility causes the stomach issues? Perhaps it is the food they serve there! Or it could be that the 35% had something different in common that causes the digestive issues, in addition to having stress - not necessarily causing the issue. Moreover, the statement is a general one. Meaning anyone with stress would then have to expect digestive issues. However, this is not given any support. It would have been more correct to say that people in mental health facilities that are stressed will tend to have digestive problems. However, this is still not a solid statistic for the problems given above.