I agree with this.
From a payment risk perspective, processing payments for someone else's business through your own PayPal account is generally a very risky setup.
PayPal (and most other PSPs) underwrite the account based on the specific merchant, products, expected transaction volume and risk profile. If the actual business activity differs from what the account was approved for, it can trigger reviews, reserves or even account limitations.
If you're considering a long-term partnership, it's usually much safer for each business to have its own payment setup. If a merchant can't obtain a payment processor, it's worth understanding why before processing payments on their behalf.
It may take a little longer to get the right setup in place, but it's far less risky in the long run.