score:4
In the circumstances you describe, your partner will need to apply for a visa https://www.gov.uk/check-uk-visa/y/india/tourism/no She should apply following the instructions here https://www.gov.uk/government/publications/usa-apply-for-a-uk-visa/apply-for-a-uk-visa-in-the-usa, the cost is £93 and typically applications are processed within 15 days (there may be an option to pay extra for expedited processing depending on where your partner is applying). Applications can be made up to 3 months in advance of the intended travel date. UK guidance specifically states that flights should not be booked until the visa has been approved. Without knowing more about your partner’s personal circumstances it’s difficult to say whether there is anything you can provide to support the application. Ideally, she should qualify in her own right without the need for a sponsor. It may be helpful to read https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/673351/Visit-guidance-v7.0EXT.PDF#page2 if you’ve not already done so.