Since they have a Business Account, one option is for them to grant you access to their account. There's a feature called Multi-User Access - the owner of the Business Account adds you to the User List, they can then create a User ID and Password for you - the User ID is "not" an email address and you cannot change the Password they assign nor the User ID - only the account owner can do that. Once that info is setup, the account owner can then assign specific permissions to your User ID which will limit your access to their account. There's more detailed info here.
After you have the User ID and Password, you can simply login to PayPal using that info and will be redirected to their account.
Note, this method is better for the folks you're trying to help because the item button code you create for them will stored on the PayPal servers making it more secure and will also be saved under My Saved Buttons.
Note, you can manually create the code they need and simply include their email address or Merchant ID in the "business" variable value however, the code is basically clear text and less secure as it's possible for someone to edit or change the code - especially the amount value.
