[@tyki]
Well I'm not sure what kinda roof it was, either way she'd have to roll over to reload her crossbow. You physically can't cock a crossbow while lying belly down. 
Even if he was sneaking silently towards her while she looked for a target and took her shot, once she rolled over to reload, she'd see him.
Again, I'll roll with it how I need to, just making sure of a few things before I post