Hello and thank you for using JustAnswer. My name is XXXXX XXXXX I will be assisting you today.
Are you still able to log in to your Facebook account? Were the messages personalized or generic?
Definitely change your password XXXXX report the event to Facebook.
Sometimes accounts are hacked and messages like this one are sent - it was likely not a person of any relation to you or your daughter in law, just some random person being malicious.
If it did happen to be someone personal, Facebook tracks IP addresses, which are addresses of your computer. If the address doesn't match, they would be able to find out where it was and take the appropriate steps to make sure it doesn't happen again. They would likely block the address from being able to access the site.
You can use the security area of Facebook to read about the different types of scenarios and file a report on the one that fits yours - you will probably have to enable your account again to be able to file.
Here is the link: http://www.facebook.com/help/?page=798#/help.php?page=420
I've seen this alot of facebook - my brother in law's account was compromised at one time too. It's frustrating...hope this helps a bit.