Well step one is to expect for nothing to change and for the animals to die. Thats most likely whats going to happen. Try to keep things in perspective. Many many animals die from neglect. Its a huge problem with fish, reptiles and rodents as they are not as loud and in your face as cats and dogs.

Next I would start by saying, "Man I really do love those gecko's if you ever think about getting rid of them let me know." If you are willing to put your money where your mouth you could change that to, "if you ever think about selling them let me know, I would give you $ for them," insert whatever you would be willing to pay. If thats not the way you want to go, then you could try "I was thinking about getting some gecko's myself so I did some reading about them on http://www.ball-pythons.net. One of the things they said you have to worry about is a respiratory infection and the way they described it makes me think your big crested gecko might have one." If your friend cares at all you can start working on getting things turned around. You could also offer to keep one of them at your place so that the sick one doesn't infect the other one. You could then setup a nice habitat in a tub or something. Show your friend how it should be done. If you are willing you could then let your friend keep the tub and then he or she would have a spare tub to put them in while the other one gets cleaned.

In the end just try to be tactful and know that as soon as you open your mouth you risk making your friend angry. I would do it anyway because any friend that would get angry is not a friend I need.