What should my daughter-in-law do when she receives such a short message?

It means that your wife's mobile phone number and personal privacy have been leaked. In this case, I suggest you contact your local communication service provider to see if you can block these numbers. In general, communication service providers can block these numbers. But ... if it still doesn't work, you'll have to change your number. Even so, there is nothing to be afraid of. You can set the SMS reception in your mobile phone, just don't receive these spam messages. It depends on what kind of mobile phone is used. You can consult how to operate the local mobile phone after it is sold.