The clarifications by the World Health Organisation and the University of Queensland states that Lemon juice would not help to fight Coronavirus.
The University of Queensland on 18 March 2020 in its 'Corona claims checked by experts' states that hot drinks with lemon and honey, vitamin supplements, foods with garlic and ginger, apple cider vinegar, gargling with saltwater etc. does not have any impact on the human immune response and would not eliminate the coronavirus.
WHO in a column of Fact or Fiction has addressed the misinformation on different kinds of remedies that were circulating in social media. It said that there is no evidence to prove that lemon, hot water, etc could kill the virus or cure a person with COVID-19.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.