+1 on the let it go. You gain nothing by insulting DD's appearance or intimating that she's jealous...rather, remarks like that reflect poorly on the speaker, not the subject.

The potential commuter is an adult, and can make her own decisions. You could let her know that all commuters face similar criticism at some point. By all means help her, but you are not responsible for her actions, and jumping in and openly antagonizing DD as some have suggested will not gain you respect.