The thing is, that photo was taken yesterday. The balloon hasn't gone down in 5 weeks; it's almost as inflated as it was on my birthday. Balloons don't normally last anywhere near this long before deflating. Either it's made of extra-thick rubber (in which case, how did she blow it up?) or her breath has some kind of balloon-sealant property previously unexhibited.
If it lasts the full 12 months, she can recycle it. Maybe that's why it's green.