So in other words, no - ngrok alone can't harm you. Althought the developer could be running a malicious site on the other end of the tunnel. The risks are the same as with any other website.
Thanks that is already helping me! Since this person said that he will help me with my scam problem and is trying to find the scammer, would it make sense to send me this link? Can he gain any information from it that helps finding the scammer?
To be honest, it sounds a bit odd that he's sending you a link to his local dev environment. Regardless of whether the developer has malicious intent, it's a bit amateurish to send you that link unless he's in the process of developing software for you.
Whilst the probability is low, it is possible to catch a virus just by visiting a website so do be cautious: https://security.stackexchange.com/questions/3848/can-you-get-virus-just-by-visiting-a-website-in-chrome
Thank you again! I decide not to trust the guy, he is definetly not developing something for me. Really fucked up to contact someone who already got scammed, just to scam them again.