Question about sin forgivness and being an accomplice in sin

Hey all!

I have a question about Christianity and about how it looks at this situation:
Let's say you are put into the situation, so I can better differentiate in the example.

You see a friend / someone familiar commit a crime.
They know you know about it or you press them about it and they tell you they regret it greatly and ask for forgiveness.
You forgive them, thinking they have changed (perhaps there is some time between you finding out, and the crime taking place, so it's not too hot on your mind).
Instead of going to the police, which would result in them being jailed, you forgive them for committing the sin.
Now, they commit another crime. Have you sinned or are you, in the eyes of God somehow accountable for it happening? You had the tools to stop him, to let somebody know of the crime, but you haven't told anybody, believing they changed, but they didn't. Have you also committed a sin or something bad in the eyes of God or Bible?

Also, would it change something, if you would forgive, but perhaps not believe, they will change? Forgiving them, hoping they won't do it again, but not trusting they wouldn't?

Hopefully it makes sense. I am wondering how would be looked at this situation, and if you went against something in the bible or any other thing in Christianity.