Thought experiment. What would you do if you could trade on an exchange with 0 fees?

Asking for a friend obviously. This friend has very little trading experience. This friend is also pretty risk-adverse and is looking for relatively safe strategies.

Let's assume you have access to a middle size exchange with all the trading pairs you can imagine, good liquidity and 0 trading fees, no matter the amount / size of trades you make.

What would be your best bet to take advantage of this?

And what kind of tools would you use to automate the trades if any?

Arbitrage / non risky strategies being a must obviously but again, this friend has very little experience and is looking for any creative ideas you might have.

His starting capital would be pretty small to get started and try things out too (>$1000).