Being a vegetarian has been proven not healthy as the vegetarians are six times more likely to have brain shrinkage than those who include meats in their diets. The cause could be a lack of vitamins such as B12 which is mainly found in meat. However vegetarians have lower rates of heart disease and some forms of cancer than non-vegetarians.
Yes, there's no reason it's unhealthy. Eating meat is unhealthy just look at all the heart disease you can get from eating it. It puts you in bad shape and makes you fat. Most people go vegetarian to help themselves loose weight and fix their bowel systems.
If you want to be a vegetarian, he have to have large consumption of eggs or milk, which as the first question stated has vitamin B12 which is essential. But other than that, it is very healthy. I suggest you do it if you want to loose weight fast or are just horrified of eating a defenseless animal.