Maintaining weight loss? REALLY UNUSUAL STORY.?

soyaaa. my friend ate only vegetables in summer last year and lost lots of weight..he use to be really fat, but after he lost weight he is back to eating regular food liike junkfood type stuff and he doesnt even play sports or workout. o.o i wanna lose liike 7 pounds. im planning on eating healthier foods and working out this summer, but if i do lose weight... can i go back to eating the good foods again, and not having to workout constantly?