Thanks for the info guys. We've booked a consultation with a tax professional tonight to talk about this and a couple of other issues.
@nun We're actually getting married in October, albeit in South Carolina due to her family being there. I don't know if that makes a difference. We intend to live in the UK.
@Kelly85 fingers crossed if we do have to file the taxes we don't have to go right back! From the documentation I've read on the SC tax site, if we're excluded from paying US tax it's the same in the state returns. For this reason I don't think she can be fined either as they would charge a percentage of the money owed, which, in our case, should be zero.
@vadio yeah, we really do need to relinquish that license, especially as she can drive over there on her UK license if we rent a car. I'll look into it. I had a look through the SC DMV site, and I can't find anywhere that says you have to be a resident. She was obviously a resident when she applied but she hasn't used the license in the UK or the US since moving five years ago. I doubt having a license that you don't use is illegal and I can't find anything on the site that says how to 'give in' the license.
In terms of her bank, I believe, but I'd have to check, that the statements do come to our home address (UK). The bank is Wells Fargo so they're all over the US, not just in SC.
I'm not sure if she's registered to vote, but she owns no property and if the bank statements are sent here and they can see it's only used to pay student loans I think it would be unreasonable to say that a US driver's license proves a domecile, especially when you consider all of her income is here, we're engaged, about to buy a house, she has a UK driver license, pays UK taxes etc. But I suppose if they're really fussy they could go against that.
I don't think she's bothered about having to file taxes in the future, I think she's worried they'll penalise or punish her for not realising she had to do them for the last five years.