Simplest way to send Ethereum

My brother just came up to me and asked my help to sell some Ethereum coins he bought a few years back. He basically showed me a piece of paper with an "Address" and "Private Key" written on it.
Neither of us have any knowledge about crypto and he just wants to sell his coins. We got so far as to verify that the coins are still in that wallet's (?) address and find some crypto exchange which will buy the coins.
Now they gave us some QR code and we need to send the coins there, but have no idea how to do so. We tried searching and found that we probably need to get a wallet app, but there are way too many options and we never could connect it to his coins.
Assuming we are as ignorant as they come, what is the simplest way for us to send those coins?