Well, in statistical terms, belle, she has an 80% chance of pulling through. Also remember that the 20% rate they gave will include things like immobility post-op and risk of chest infections, and so on, not purely the op itself. This is good news, because if she can be motivated to really work at getting up and about, co-operate with the use of TED stockings, take the pain relief so that she is willing to mobilise, etc, then the risk will reduce.
Is she having it under GA? They could explore the option of a Spinal Block with Sedation rather than GA, which would reduce the risk slightly.
The fact is she HAS to have the OP, doesn't she? A woman with COPD, heart failure and kidney problems is more likely to die if they are bedridden, which means that she doesn't have an option about the OP, in real terms.
I know it sounds scary, but 20% isn't a desperately bad chance of dying, in the scheme of operations. And to be honest, whatever the odds, if you are the one, you are the one, the other 4 don't count.
It is a worrying time. But please be reassured that the stats aren't terrible. If 1000 people have the op in her condition, 800 of them would be here to tell you all about their improved quality of life.