Shaykh Ibn Uthaymeen was asked:
“There are some brothers who are callers to Allah and work in the arena of nursing. In this arena there is free-mixing. Should they call the men to Islam only or the men and the women [should call to Islaam]?
“It is obligatory upon the people to call to the religion of Allah, the Exalted. However, a man speaking with a woman, even if he is calling to Allah, is a cause of fitnah. So what is more deserving is that there is female nurses who call the women to Islam and men who call the men to Islam so that there is no fitnah. This is because Shaytan circulates through the son of Adam like how blood circulates. So sometimes when speaking, Shaytan enters between a man and a woman. So every speech has its place.
As for if there are women who are gathered and a man stands and gives a speech, then there is no harm in this. However, giving da’wah directly to a woman, then we do not see there is a benefit in this. Even if there is a benefit, the harm is greater. As for the male nurses, then call them to Islam. However, if you fear they will leave, then one by one. Call one of them to your house, pour some coffee, treat him well, and expose him to Islam. As for the one who is capable of calling to Islam and does not, I fear he will be sinful for that” End translation. [Liqaa al-Bab al-Maftooh no. 37]
Faisal Ibn Abdul Qaadir Ibn Hassan