Yes, Fenbendazole is effective against hairworms (Capillaria sp). In regard to the question you asked about small pieces of bread soaked in buttermilk, that is merely something I've done for birds which had crop impactions, or intestinal problems which were remedied, and wanted to get easily passed nutrients in them. The protein, calcium, vitamin A,C & D with saccharides/ lactobacillus helps, and when given in small amounts, seems to get them encouraged to desire more food, then move on to moistened feed, and eventually to dry crumbles/pellets again. The bacteria of the buttermilk attach to the intestinal mucosa, thereby forming a physical barrier that blocks the attachment of pathogenic bacteria, then nutrients from other foods can again be utilized. Commercial producers use milk replacers for a limited time to contend with early signs of enteritis in flocks.
Sorry about your hen, Scott. I've found worming birds 2-3 times over the period of a year is a way to prevent many intestinal related health problems. It is when people are too frequent with medications, or use the same medications consistently, that they become ineffective. It is always good to keep two types of dewormers( i.e. Fenbendazole, Albendazole, Pyrantel), Corid (Amprolium), Sulfadimethoxine, vitamin-electrolyte-probiotic powder (Probios has an excellent profile of good gut bacteria), an effective emulsified concentrate insecticide labeled for use on poultry as well as premises, and antibiotics such as Neomed325, Amoxicillin, or even Chlortetracyclnie in water dispersible powders in the poultry med cabinet.
Probiotics naturally occurring in food stuffs may not have enough of what benefits the birds. Probiotic dispersible powders like I mentioned, are formulated for and better utilized, in addition to being stabilized for longer shelf life when stored properly in a cool, dry place. Here's some information about probiotics:
http://www.thepoultrysite.com/artic...poultry-and-pig-nutrition-basics-and-benefits