Hello,
my advice would be to try ligher balls. If your wife has weak fpelvic floor, the 78g of these may be just too much for her to handle.
Size and weight are the most important things to consider when buying Kegel exercisers. Generally, with weak mussles you should start with lower weight and preferably smaller balls. And make your way up. LH does a very good job with providing how the different exercisers weight and what is their size. Look for smaller ones, I would say bellow 60g to start with. I personally have the lelo Luna beads. I know they are more pricey, but well worth it. Basic instruction manual if you are new to this. And you can customise your Kegels to suit your level. From single ball to two heavier balls inserted at the same time, which weights almost as the Tracey Cox.
http://www.lovehoney.co.uk/product.cfm?p=12689
Also to start with, she can just practice without anything in. She just needs to squeeze her inner muscles. And hold them for maybe couple of seconds to start with. Repeat few times, preferably few times a day and increase the lenght of how long you can hold it or the number of times you do this.