Is it obligatory to return the greeting of a non-Sunni female colleague who sent greetings to check on the questioner's well-being, given that it is unknown whether she holds apostatizing innovations or whether there is the ability to denounce her innovations or invite her to the truth?

Innovators (Ahl al-Bid'ah) are of two types: one whose innovation is an act of disbelief (kufr), in which case he is treated like a disbeliever. He should not be greeted first with salam, though it is permissible to return his salam if he initiates it. The other type is one whose innovation is not an act of disbelief. In this case, the general rule is that it is obligatory to return his salam. However, it is permissible to shun him and refrain from returning his salam if there is a شرعية (Shar'i benefit) in doing so, such as deterring him from his innovation. But if shunning him would only lead to greater corruption and further indulgence in the innovation, then he should not be shunned, because he is a Muslim.