This is a very wrong reason to stay married to someone. You can’t be with someone simply because you feel sorry for them. I mean, you can, but it’ll never be a healthy relationship.
It’s always better to be honest with your partner. The wife will be hurt, and she’ll suffer… That’s inevitable. But, the truth is, she would suffer even more if she found out that her husband is staying by her side only because he feels sorry for her.