Thanks for the advice folks.
The buyer came back and said that Hasbro Pulse customer support refused to offer any support because it was bought through EB Games. The buyer is asking for a partial refund.
I've emailed Hasbro Pulse Customer Support and EB Games Customer Support myself now to ask for a replacement part. EB Games obviously can't supply the part, but if they wanted to they might be able to reach out through their Hasbro Supply chain, because AFAIK, they are an authorised reseller of Hsabro for Aus.
I may also reach out to Hasbro Australia and see if they can do anything.

After PayPal and eBay fee's, I'll probably be lucky to break even on this so I'm reluctant to offer any kind of partial refund. I'll see what I get back from Hasbro myself and then talk to the buyer, see what they expect.
I can definitely understand their disappointment and have been in a similar position myself but there's a reason I explicitly wrote on the listing that the product was sold as-is and there would be no support or warranty. If they couldn't abide by that, they really shouldn't have bought it.
However, I am trying to help as best I can.

If they get completely unreasonable, I'll just tell them to ship it back, and then refund their purchase price.

Geez I hate HasTak. Takara has dropped the ball majorly with recent Masterpiece releases, and Hasbro's retail items have had a bad reputation for years. This kind of crappy lack of QC and refusal to offer Customer Support though... no excuse for that. Hopefully the buyer just messed up the contact with HasbroPulse Support and I can get some kind of remediation.