Good evening chajergeay and welcome to the NS Community.
I'm not sure if the Revolut payment option would work, as far as I'm aware subscriptions can only be paid through an iDeal payment which requires a Dutch bank account.
That said, if you already placed an order where you tried to do that, but cancelled it, that order is now stuck in our system. Over time (usually overnight) it will automatically get cancelled, at which point you can place a new order. However, I can imagine you would like to have this resolved sooner.
I would recommend for you to contact our Customer Service. They can manually cancel a failed order so you can place a new one sooner, but they can also help you with the purchase of a subscription without a Dutch IBAN. They can be reached by phone, Social Media and Chat, although I currently wouldn't recommend Social Media if you need it resolved quickly. You can find the contact details here.
I'm not sure if the Revolut payment option would work, as far as I'm aware subscriptions can only be paid through an iDeal payment which requires a Dutch bank account.
That is not entirely correct. Ordering subscriptions online still requires a payment via iDeal, and although it's mostly Dutch banks offering that, iDeal is not limited to Dutch bank accounts only.
Revolut offers iDeal payments. I have already used it to pay for an NS subscription, so that should also work for someone else.
The order being stuck sounds like the more plausible cause here.
If your friend already has a personal ov-chipcard it should be no problem but the last couple of days I’ve seen the strangest errors when ordering a subscription in combination with a new card.
Contact NS Customer Service regarding the open payment error.
The OV-fiets subscription can also be purchased at a service desk on a station (personal ov-card needed).
If you already have an OV-fiets subscription, it’s possible to rent two bikes (when travelling together of course).