Hmm… Dark grey area…
You can’t actively scout saying that you will trade on someone else’s account. But if someone has shared his login credentials and password, and you both have some kind of understanding, no law stops you from trading on that account.
The only issue with such a setup is, if there is a loss made, the owner of the trading account can turn sour and try taking different steps to recover losses from you.
Dude, its like you are driving someone else’s vehicle. Everything is ok and legal I guess, does anyone/or any authority stop you. No. It’s fine, until you mess things up. Hope you got it. But it’s always a good idea to stay away from other persons account, unless they are very close to you.