In 2010, you came across a site that was giving away bitcoins. The site said it was free money so you signed up. You were logging in to figure out how it was money but couldn't. The last time you logged in you had more than 100 bitcoins. Later you forgot about the site until 2017 when a ponzi was advertising Bitcoin. Bitcoin? The name rang a bell and you stated hearing stories. You remembered you must have many bitcoins somewhere but you can't even remember the name of the site. How would you recover your BTC?
We don't even know if you are talking about the wallet or website And where did you get that 100 BTC you are claiming.
If it's stored in a wallet then you should have a phrase or private key to recover the funds if you don't have that then there is no way you can recover it unless it's online wallet then you just need to sign in your wallet.
I personally think that it depends on whether the website was legit or not, you should just try remember the name of the website and google it, If you find any results then proceed to recover your account with the email address you registered with. I assume you are referring to faucets, most didn't look legit, I am doubting they will survive from 2010 until now, also doubting chances of you recovering your 100 BTC.
For almost 10 years if that was from faucet then the website don't give any more rewards that high now and the balance he forget has been use already to send rewards to other that time. If I were him I will send it to personal wallet than website. Better for him to forget about it than regretting for whole of his life , since that 100 btc is a large amount.