A few questions I’ve had lately are around why customers can’t use Copilot Pro (US$20/month) rather than Copilot for M365 (US$30/month).
Copilot Pro is designed to be attached to a M365 Personal or Family subscription, so a non commercial one.
As you can see from the above chart, Copilot for M365 also adds in Teams copilot functionality (which is going to be a key function in businesses) and also Data Protection (more info here Data, Privacy, and Security for Microsoft Copilot for Microsoft 365 | Microsoft Learn).
So in short, they will need to buy the Copilot for M365 for use in a business.