Girls, hello ? My nephew has had a terrible diaper rash for two months…

Girls, hello ? My nephew has had a terrible diaper rash for two months now... Sister's doctor is on vacation and won't answer calls... What should I treat it with? What can I apply? She ate some bad food and the baby broke out in rashes ? Everything was fine before that. Thanks in advance. And yes, she won't eat any more sweets after this.