I use Paypal all the time and as an unregistered person you can indeed do a one-time checkout without account. This essentially turns it into a credit card system for my business. Now, if the email you are using has been used before on the paypal system, it will ask you to login.
But not all systems support this and whatever checkout she is using may not work this way. The easiest way to do a "guest" credit card payment is to have the seller invoice you through Paypal.
And yes, another diet is a great choice.