Does one who was aware of a young man and a young woman engaging in an illicit relationship still bear a burden after repenting, ending his relationship with the girl, and calling the young man to repent?

What you did by spreading evil deeds is a great sin due to the unlawful things that result from it. However, you did well by sincerely repenting, which erases sins no matter how great, as Allah Almighty says: "Indeed, Allah forgives all sins." And as long as you have repented and explained to your friend that his action is unlawful, there is no burden upon you. We advise you to continue inviting your friend to abandon disobedience if you feel safe from temptation; otherwise, leave him be.