he only has to include it with higher fees
Should use equal fee fee rate, if the purpose is both transactions are included in the same block.
If he's talking about an addr that's been dormant and he just wants to monitor where the funds are being sent, I don't think it has anything to do with whether they're mined in the same block or not; the blockchain already handles that.
But if he's monitoring the dust transaction for other purposes and wants to include it in the next block together, it's not really a question of setting the same fee being the most effective. Instead, it needs to be a better-developed bot that quickly builds the dust transaction dynamically based on the size of the transaction being monitored. The fee depends on whether the bot assumes you have a fast parameter or if it checks every x seconds.
If it doesn't monitor in real time, setting more fees gives it a better chance of both being included equally.