As so many other have said...just be friendly!
It's wonderful that you are thinking about her!
I've been that 300+ pound person in the gym before. More than anything I just appreciated people being friendly and small talking.
I'm a member of a fairly small gym and make a point to chit-chat with everyone, especially new women that seem like they could be having a difficult time getting started. I've had many people strike up conversations with me becuase they've seen such a change in me over the past 9 months. I love talking to people and hope that one day I can help, inspire and motivate others.