Is a Muslim sinful for helping his Christian colleague go to church or for advising him on personal matters? And what is the guideline for cooperation between a Muslim and a non-Muslim?

It is permissible to interact with the disbeliever and benefit from him in worldly matters. However, befriending him is prohibited except with the intention of inviting him to Islam. Let your intention in dealing with the Christian be to win his heart over to Islam. Cooperation with non-Muslims should be in righteousness and piety, not in sin and aggression. Thus, you may show kindness to him in studies or by giving him a ride home. However, it is not permissible to give him a ride to the church or assist him in a forbidden relationship, and you should gently forbid him from such. It is not permissible to win his heart by encouraging him in forbidden acts. Indeed, your refusal to cooperate with him in forbidden acts may be a path to his guidance.