I can pretty much assure you 100% that the infection was caused due to contamination during the blood patch. I have personally placed a few dozen of these and can tell you that unless absolute meticulous sterile technique is followed there is a high likelyhood that you will wind up with skin bacteria being injected into the epidural space along with a nice 30ml's of blood for it to grow in. Most pain docs were at some point in their life practicing anesthesiologists so they tend to be very careful to avoid contamination, that said I have witnesses quite a few that are just plain sloppy, doing things such as drawing blood out of an existing IV for the patch or simply swabbing the skin with alcohol prior to drawing the blood. What you need to do is sit down and write down every single detail about the procedure that you recall in order to best determine if any standards of care were violated. Chances are that somewhere in your husbands medical record is a consent he signed that states "risks were discussed" so that argument will basically get you nowhere.
You also need to be prepared for the fact that despite your husbands ordeal no actually negligence occured and this was just plain bad luck.