I think Rick was right in not letting her come back, sure she was trying to save others, but she should have consulted the counsel that they had for big decisions.. That's the kinda thing it's there for. Not to mention, if she came back she would most likely be the next to die when tyreese finds out. This way Rick can tell tyreese he dealt with it.
I think there is something more to the hippy girl and the guy with her, i think we will be seeing him again with another group, and that perhaps the girl was having second thoughts about it so he cut her leg off and made it look like Walkers got her because of her bad leg. So he could continue with the plan with out anything happening to it.. Just my suspicion