NopCom v.2.5 One Page Checkout - Redirect to PayPal (PayPal Standard Plugin)

I have had an issue reported to me today by a customer who successfully placed an order but failed to pay.

They claim after they click the confirm button "the paypal page said the merchant address was invalid so I was unable to pay".

I tried to get further information - they said: "The paypal page that allows you to input payment details didn't even appear to give me an opportunity to put in my payment details so i could not even make an attempt to pay!"

"Sent from my iPad" was in the footer of all their correspondence. Using Firefox with the "Default User agent" switched to IPhone 3 I successfully placed an order and was redirected to PayPal as I would expect. I do not have access to an IPhone or iPad, just Android & PC so cannot test the real platform.

This is a worry – I know there's not much to go on but any help is greatly appreciated.