next time they witter on about being so busy - ask them what they are doing - if you can help? Then they will be put on the spot and perhaps in the future they won't keep going on.
Or - you could say how busy you are - and say how lucky she is not being so busy then again she would have to justify herself - not easy if she really isn't busy.
Although people cope with work in different ways, something you find easy she might feel pressured and stressed out by. Sometimes I can be doing something and it will all fall into place nice and quickly other times everything is awkward and that stresses me - she may have the same thing - and no one really knows everything someone else is doing!