Any individual who has breached the conditions of their stay, or a previous stay in the UK, or who is found to have entered the UK illegally may be refused entry clearance or further leave to remain (for any purpose) solely on those grounds. It is therefore essential that individuals comply with their conditions of stay, and seek legal advice before submitting further applications if they fall into any such category.

 

Not all the doors are closed!

She could come back,you can always explore EU angle,work and live elsewhere and come back to UK.
She is outside the law ,therefore I would recommend seeing solicitor before you take any action.

 

No problem there.

 

GForce - under what visa was she here initially (e.g. which one did she overstay)?
She probably needs to seek legal advice about her current status, but there is a kind-of-loophole.

If she goes back to US and you enter US on a visitors visa and you then marry there, she can apply for a spouse visa to go back to UK.
Because a spouse visa is a "settlement visa" they are supposed to consider the application in a different way to how they'd consider a non-settlement visa.

EG so she has overstayed by 2 years.
If she applies for a visitor visa or student visa or other kind of non-settlement visa, then it will probably be denied in light of her previous overstay.

BUT if she applies for a settlement visa category, then they are supposed to overlook the overstay because it won't be applicable to the conditions of the new visa. The new visa would not expect her to leave at the end of it, she's hoping to settle in the UK, so hopefully it wouldn't be counted by the immigration officer reviewing her application for spouse visa.

Note : I am NOT a specialist in immigration matters, this is merely something I have read on other boards.

Any overstayer should get legal assistance before making any movements.