How can one deal with a friend who is masculine but speaks effeminately due to being raised in a female environment, and how can one advise him without embarrassing him, especially since he is unaware of people's perception of him and does not know how to respond to their taunts?

If your friend imitates women in his speech and movements, you should wisely warn him against such imitation and explain the Islamic ruling on it, as the Messenger of Allah, peace and blessings be upon him, cursed men who imitate women. You can do this in a way that does not hurt his feelings, such as explaining the ruling without directly addressing him, or by sending a message, or gifting materials that clarify the matter. Make much supplication for him, and be keen on connecting him with righteous company.