Public Protector Busisiwe Mkhwebane's impartiality has been called into question after a former minister she is supposedly investigating Mosebenzi Zwane was among the guests at her 50th birthday party.Zwane, a former mineral resources minister and Free State Agriculture MEC, reportedly attended Friday evening's party with fellow ANC MP and corruption-accused Bongani Bongo.READ | 'Shameless' - Public Protector criticised for celebrating 50th with 'dodgy' guestsOther well-known political figures at the party included Ekurhuleni mayor Mzwandile Masina, who recently said he would go to jail for former president Jacob Zuma, former national chairperson of the EFF Dali Mpofu, the African Transformation Movement's head of policy Mzwanele Manyi and former senior NPA prosecutor Nomgcobo Jiba, who was fired by President Cyril Ramaphosa in April last year after an inquiry found she wasn't "fit and proper".Zwane was allegedly a central figure in the Estina dairy farm project, which was the subject of testimony at the commission of inquiry into state capture.Estina, the company charged with running a farming scheme to benefit black farmers in Vrede, Free State, was allegedly a Gupta front with government money allegedly funnelled to and being laundered for the Gupta family.READ | Parliamentary process to 'dethrone' Mkhwebane to continue despite court challengeIn December, a British investigative NGO claimed that Zwane allegedly received benefits worth R500 000 from Gupta companies a month after he signed the 99-year lease with the Guptas for the Estina farm, City Press reported.Approached for comment, Accountability Now director, Paul Hoffman, SC, told News24 it is "extremely inappropriate" that Zwane attended Mkhwebane's birthday celebration.He said that someone she was investigating receiving her hospitality, created an impossible situation."It is another indication that she doesn't have a cooking clue of what she is supposed to do," Hoffman said.'Highly irregular'Accountability Now recently brought an application to have Mkhwebane declared unfit for office, which will be heard in the Gauteng High Court in Pretoria on Tuesday.Executive secretary for the Council for the Advancement of the South African Constitution (Casac) Lawson Naidoo said it was highly irregular for Mkhwebane to have Zwane at a very personal event like a birthday party and it brought her office into disrepute.He said it was her responsibility to be perceived as impartial, and actions such as this compromised that."It is unbecoming of a Public Protector to behave in that way." Naidoo said one would have expected Mkhwebane to act with more restraint given that a Parliamentary removal process against her is under way.Professor Deon Rossouw, CEO of the Ethics Institute, said it was not per se unethical for Mkhwebane to have Zwane at her birthday party."It is more a matter of professionalism and being seen as being objective," he said.On Monday, News24 reported that Mkhwebane's spokesperson, Oupa Segalwe, said there was nothing untoward in terms of who was invited to the party."The guest list for advocate Mkhwebane's 50th birthday party comprised politicians, people from the legal fraternity, colleagues, family and friends," Segalwe said."Among the politicians who were invited were President Cyril Ramaphosa and Deputy President David Mabuza. Ms Jiba comes from the legal fraternity and is under Pabasa [Pan African Bar Association of South Africa], which is a project the Public Protector supports for the transformation of the legal profession."Mr Bongo, like the Public Protector, comes from KwaNdebele. Mr Zwane came with Mr Bongo. Accordingly, there was nothing wrong with the guest list."Ramaphosa was also the subject of a contested Public Protector investigation into his ANC presidential campaign funding.Accountability Now's application against Mkhwebane meanwhile will be heard in court in Pretoria on Tuesday.