It's worth checking or encouraging them to check, partly because unlicensed HMOs are a safety & welfare hazard, but also because tenants can apply for a Rent Repayment Order if it turns out their property is unlicensed.
Some London councils are swamped with unlicenced HMOs because the landlords know councils don't have resources to take enforcement action. Every council has an HMO license register on their website, so it's easy to check.