My friend heard about the Ethereum blockchain and thinks it's the safest place to hide stuff.

Prove him wrong.

Network : Ropsten Test Network
Contract Address : 0x94f23F189583e828E73573905A19d93E21D0E957

they gave. eth address and said its network as ropsten test network

open https://www.myetherwallet.com
change network to Ropsten test network
go to View Wallet Info
and give the address and check last transactions
https://ropsten.etherscan.io/address/0x94f23F189583e828E73573905A19d93E21D0E957

https://ropsten.etherscan.io/tx/0x51fb0720ce718920573882821a2ca39722c42f1d49ed460e6912bb0fff5562c2
check the Input Data:
click on view input data as and aelect UTF-8
u see the flag

d4rk{sucH_s3cr3cy_much_w0W}c0de