First Ted, I have seen several wounds with the type of drainage you describe due to fat necrosis and it can go on for some time.
Second, better out than in, so opening up the wound to promote drainage, is always appropriate.
Third, I have frequently had clients who have showered with thier abdominal wounds. These people live in the projects and drink our city water, which a family friend has long described as "Chlorinated, urinated, flourinated, city water." We have never had a pt develop a complication due to this type of irrigation. What would be worse would be a bath, being upright in the shower allows gravity to do the irrigation. I am sure it seems gross b/c of the iron, but I am sure since your wife has had exposure to this water in many ways for some time, her body will not react to it in a harmful way.
Bottom line, it sounds like the wound care is appropriate. But, if you are uncomfortable with it, you could certainly use a syringe and irrigate the wound with NSS if you wanted to instead. Nothing wrong with that.
On a final note, the only time I have ever seen a pt who had a wound of this kind that just wouldn't heal that last little bit is when, not to scare you, but there was something left inside the cavity. We had a woman on service for 2 months like this. Finally her primary nurse asked a wound nurse to see her, the wound nurse really probed into the tiny hole and pulled out some gaggable long-forgotten peice of packing about 3 inches long. From the hospital dressings I like to think. But, it could have been from one of our nurses, who knows. Anyway, she pulled this gagger out, the wound healed, problem solved. Had another pt with a simialr story, a sponge left inside. It does NOT sound like this is your wife's problem!! I only mention it b/c you should think about it if the drainage doesn't stop after about 6 weeks.
I had one man who had his ear removed from cancer, and one wouldn't think you had too much fat on your head right? Well, that fat continued to liquify for 2 months before easing back. It did eventually, and the pt was fine.
Hope your wife is feeling well.