hey, and welcome to 'ology!
adding muscle will make you more compact and dense, which will firm you up. it will also make you leaner, since muscle cells take up less space than fat cells. just think: would you rather be flabby or tight? contrary to popular belief, it isn't that easy for women to get huge and bulky (thus the use of AAS in some female bb'ers). hope that helped! try lifting for a month or two; you'll be amazed at how much better your body will look!
~MINX

i agree with strnghmnx. it is hard for women to get big and bulky. i myself am a bodybuilder and i have a hard time holding size without the use of supplements. adding muscle will make you look tight and lean. just tell your girlfriend to look at the women who do figure and fitness. they all do weight training. she doesnt have to use heavy weight. tell her high reps light weight. good luck!!