A saying says "Don't make promises when you're happy." I would say " Don't ever make promises whenever, wherever just to comfort someone."
To avoid promises being broken, just don't give any, don't talk because if you talk and say promises today just to make someone happy, you will regret it for the rest of your life. The moment he have heard what you said, he will cling unto it and keep it as a fuel to his hope.
If a promise is told, it is futile when no action is taken. As Mae West said "An ounce of performance is worth pounds of promises."
Please, don't make promises you can't keep.