Let them go. They are not going to get far without money. They will figure that out and come back. At 21 she should have a plan in place; however, it sounds like they are just naive about what it takes to survive. I'm sure you will get a call within a few days of them leaving that they are either coming back or need gas money to get back. The other thing is, she has to figure out that her boyfriend isn't a provider on her own. Don't push her to leave this boy; it has to be her idea. She's an adult, and you need to look at her decisions that way. At some point, she has to become responsible for herself.