I had to share this with all parents...my nephew had a bath last night and mixed 2 shampoos together (as he likes making potions!) and washed his hair, it has burned all his head and he has burn marks down his face where it has dripped, it is now blistering all over....I knew that you shouldnt mix xleaning chemicals but there are no warnings on the bottles about mixing shampoos....just wanted to tell everyone as a warning, never mix shampoos!
Don't go in all guns blazing lil just give them the facts along with a photo and they will probably ask for samples of the offending shampoos for analysis. Remember this is how you get products recalled so you have a duty if you like to other consumers plus you will no doubt receive some recompense for your nephew's discomfort.
Can you be absolutely sure that he didn't add anything else to his "potion"?
As far as I know, most shampoos will contain roughly the same ingredients, it would seem unusual that making two together would cause such a severe reaction unless he has a severe allergy, which you would think he would have to one of the individual products as well.
Anyway, hope he gets well soon and that your sister gets to the bottom of it. Please keep us updated.