It's not just about gambling. It's a human nature thing. It's not the same for everyone, but some people are like that. Negative effects can be seen on the body when too much stress is applied. These effects can occur because gambling creates a very stressful ambiance. I hope your friend will not be exposed to too many stressful events other than gambling and his health will not deteriorate.
But I have seen people who gain weight and become insomniac when they are stressed or over worked.
If not early but people start noticing that and there are body changes which are disturbing the lifestyles of the people