Should we stop teaching Arabic to an engineer who is one of the people of Bidʻah, or continue doing so as a call for him to be guided, despite the suspicion that he might be practicing taqiyyah?

Inviting the engineer to the truth and clarifying his deviation from his madhhab (school of thought) is a great act and a noble effort for which a Muslim will be rewarded, whether the engineer responds or not. This is in accordance with the Almighty's saying: "And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful." (Quran 3:104). If he responds, the reward is even greater, as the Prophet (peace be upon him) said: "For Allah to guide a single man through you is better for you than possessing red camels," and "Whoever calls to guidance will have a reward like the rewards of those who follow him, without diminishing their rewards in the slightest." Therefore, it is advisable to continue teaching and inviting him, as long as the caller does not fear being influenced by him.