Could one sue a health care provider or clinic for misdiagnosis?

My cousin was given penicillin by a Rediclinic healthcare provider for an ear infection she never had for the past few weeks. Now she developed a drug allergy from it. She was also warned about the possible side effects from taking penicillin during her first visit with said provider. She recently went to a different clinic to take both antihistamine and NSAID for her drug allergy. The kicker? She was also told by the new provider that she didn’t have otitis media at all and apparently just have something else going on with her ear. Now she couldn’t focus on taking care of her ear situation for a while because of her drug allergy. She had to take time off from work since she finds it so uncomfortable to barely be able to sit down in a chair with her hives.

So I wonder if my cousin might be eligible to sue the Rediclinic provider for misdiagnosis? I read up one article that suggested there could be a possibility to present this scenario as medical negligence over penicillin allergy.