No contraindication for gluten or coeliac disease for co-amoxiclav in BNF Roger, but diarrhoea is a possible side effect on just about every drug you can take, including co-amoxiclav. Given the duration of most drugs, particularly antibiotics, tends to be short, I suspect the presence of small amounts of gluten will have minimal effect. Not knowing your wife's level of sensitivity however that is only a best guess. Coeliac disease is a broad spectrum condition and can affect people to different degrees. It's clearly something you are concerned about so I would suggest speaking to your GP (or whoever prescribed the co-amoxiclav) for reassurance would be the logical step, rather than trusting to internet sources. The good news is that, as I am sure you are aware, exclusion of gluten from the diet relieves all symptoms, so if gluten is present you need to weight the possible coeliac related effects against not taking the antibiotic if no alternative is avalilable that is gluten free. Given antibiotics are generally prescribed only when necessary I would recommend continuing to take the course as prescribed until you get conrifmation of your query from a reliable medical source.
HTH,
Steve