This may make me a bad property manager but I always lean on the side of kindness. I know... I know... this is 'business' and 'contractual agreements'... but it really is all about people and profitability.
When a person is already going through something traumatic like a divorce, family death, or other emergency I always try to make sure that their rental situation isn't just another stress on their life. I do inform them that they will not get their deposit back but I will do all I can to make their situation better by letting them move out when they need to.
I love my approach (which I have been using for the past 10 years) because when you talk to someone and tell them not to stress about their lease their entire demeanor changes and a bit of relief hits them. They will be more than happy to return the favor by getting the unit as clean as possible and ready for the next tenant. Now, when they share the story with other they will tell everyone how great the property manager was and how thoughtful they were in the situation.
Also... this approach make the best financial sense. If you try the intimidation method (aka 'I could make you pay $XXX for breaking the lease) you put the tenant on the defense and they will not going up and out of their way for you... they will do just want is needed to avoid your threatening hammer of fines. It also may spark some tenants to go to the extreme and end up damaging things on the way out because they feel like they were miss treated.
So... be kind and treat people like people, we all go through tough times. In the end you will make people's lives better and you will save more money in the long run.