Will the husband be sinful if he gives free circus tickets to his friends and colleagues (Muslims and non-Muslims) who ask him for them, knowing that they might see evil there? And does the ruling differ if he gives them to Christians without Muslims? What is the advice for him?

The circus, in its current form, is not free from many شرعي (Sharia-compliant) prohibitions, such as magic acts, the baring of 'awrat (private parts), and forbidden mixing (of genders). Therefore, it is haram (forbidden), and it is not permissible to call to it, participate in it, promote it, or assist in it, because it falls under the category of cooperating in sin and aggression. Allah (the Most High) says: {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty} [Al-Ma'idah: 2].

Your husband is sinful for giving out the tickets. The forbidden cannot be justified or aided by anyone. A Muslim must invite people to guidance, goodness, and what is lawful. You must remind him of Allah (the Most High) and that by assisting in this, he angers and displeases Allah.