well 由其should be尤其, it means "in particular" so let's translate it: Friend (A) says she is thinner because she isn't sleeping well. Friend (B) tells her she mustn't get thinner because she is already too thin. Friend (A) agrees and says: in particular that part, you know that. (the boobs or the butt, the part Chinese girls care about very much but they are shy to say it out) So I think now you can understand 做那事 "do that thing"

More information about 你懂的：It is said that .......你懂的 had originally been internet slang, meaning '........... , you know what I mean' . The speaker actually was meaning ' you know what I want you to send me is the certain pornopictures or pornovedeos' ; but later it has come to be a common phrase meaning ' i think you know what i am talking about'. It is pretty popular recently.