I know what it's like to break a leg - I shattered mine in 2009 and it took me nearly 8 months to walk again without aid. I packed on the weight during this period too - it was hard being away from my horses literally being unable to do anything for myself. Naturally I became depressed which looking back on it now, frustrates me because I ended up making things harder on myself. Thankfully I snapped out of it!
Is your friend getting physiotherapy? Keeping her leg moving is so vital so she doesn't lose muscle and tone. Unfortunately my doctor gave me conflicting advice and I followed his first advice and lost all use in my leg which is why it set me back so much. Physio - and lots of it in my case - is vital for aiding the strengthening of the weakened limb so she can get back to the things she enjoys.
I second not making a big deal about her weight. It's difficult because you're so worried for her, but if you become too focussed on it, she may find your interest a negative thing. I hope you get me there.
Have loads of fun, and like I said, I hope you're aware of just how lovely you are - most of my "friends" saw me as crippled and so avoided inviting me anywhere because I would "hold them back".