Yes, a third party can make a payment to your account. PG&E is not able to provide any account information (i.e. account number, customer name, last 4 digits of the social security number, account balance, etc.) to any party other than the customer of record.
If a third party wants to make a payment by phone, they should call 1-877-704-8470.
If a third party wants to make a payment online, they can click the One Time Access button that appears next to the Sign In button on the Your Account login screen and follow the prompts.
In both scenarios, the third party must have the account information to submit a payment and that information must be supplied by the customer of record.