Oh wow...that would really be difficult. If they are having all these cosmetic procedures, then obviously they already feel bad about their appearance, and don't accept themselves...you don't want to pile on, but at the same time, you don't want to encourage further 'work', or make them feel like they need to 'fix' what's already been done.
If I was in your situation, I think I would make sure I was alone with the person when I talked with them, so that there wouldn't be any fear of being overheard. I would emphasize your love and care for them, and how you've always loved their face. Let them know that the signs of aging didn't bother you. And then gently say that they look 'different' now. Not better, not worse. Not freakish (I hope). That anybody that didn't know them would not be surprised at their appearance, but that for you, and for other people in your circle, this is a big change, and it will take a while to get used to it.
Then let them know that you still love them, and that you know that you will soon be able to reconcile the change in appearance. Perhaps, you can ask to watch them while they sleep or nap...spending 30-45 minutes studying their face can really help your brain to 'catch up' to the new face. Try to emphasize that the most important part of their beauty comes from within.
I hope that a heartfelt conversation like this is enough to dissuade them from undergoing any further cosmetic procedures. You're right that lying to them would be wrong, and possibly have worse consequences down the road.