Is a colleague's driving to the institution to obtain a sick leave form considered among the matters that arouse suspicion, especially since the husband views this as trivializing the matter?

The criterion for evaluating matters in Islam refers back to the Qur’an and Sunnah (definition), not to people's opinions. The interaction between non-mahram (definition) men and women in Islamic law is based on restriction and blocking the means [to sin]. There is no harm in working in a mixed-gender institution if there is distance [between genders] and adherence to Islamic manners. As for handing a paper to a non-mahram man without privacy (khalwa (definition)) or suspicion, and with adherence to the hijab (definition), there is no harm in it.