Hi Ann, just wanted to chime in. Former C Differ here too, and it is my understanding that of the two antibiotics used to treat C Diff (flagyl and vanco), only Flagyl is effective via IV. I dont know anything about H-pylori, but if flagyl is used to treat it - that may be an option...best of luck to your father.
It doesn't matter whether he takes the antibiotics by mouth or by IV. They're going to end up acting on all of the tissues/systems of the body in the same manner - via the bloodstream. The antibiotics that are taken for H. pylori don't work on the bacteria in the stomach itself. They act by attacking it via the bloodstream. There is only one antibiotic I know of that has a slightly different mode of action and doesn't seem to need to enter the bloodstream to work, but I don't believe it works on H. pylori.
Your father does have to be very careful, however I'm wondering if there aren't some antibiotics that have a lesser chance of resulting in C. diff but are also effective on the H. pylori? Have you checked up on that? And is your father supplementing - or did he supplement - with probiotics while and after he was being treated for C. diff the first time? Probiotics are thought to help keep the quantity and quality of the bacteria in the GI tract fairly stable, and many docs are now using them during treatment for C. diff and afterwards. Many people who unfortuantely have to take antibiotics hopefully rely on probiotics before and during the course of treatment to try to lessen their chances of C. diff - although I don't know of any studies yet that say they definitely help.
There's a group of people who have had C. diff or are still battling C. diff and the group has a lot of good information on the subject. They are a (.org) group, not a (.com) goup. You can check them out at cdiffsupport.