What is the ruling on a wife communicating with her male colleague at work via WhatsApp to choose a gift for her husband?

The husband must make it clear to his wife that the male colleague, like other non-mahram (definition) men, is not permissible for her to speak with or message without a valid need. She must cut off communication with him and with other non-mahram men without a need. The general principle regarding a woman's work is that it should be in a place where men and women do not mix. If she is compelled to work in a mixed environment, she must observe the Shariah guidelines and limit her interactions to what is necessary. If she becomes negligent, the husband must explain to her the seriousness of this and guide her with wisdom. He may even prevent her from working if necessary, knowing that the default is to think well of his wife and that spying on her is not permissible.