Just ask her to go to the US embassy to apply for a visa.

They don't ask for tickets or hotel proof. I am not sure if a letter of invitation from you would be necessary. It might, in fact, lead to more questions about her relationship with you.

If she is working in the U.K., and has a letter from her employer, and of course has enough money for the trip, I think it should not be difficult to get a B1/B2 visa, especially if she has an ILR status in the U.K.

She will need to ring the embassy to make an appointment. Now it's summer and they are very busy as usual. If she's lucky, she may get an appointment for end of August.

She needs to go there in person on the scheduled day, and like everybody else she will be interviewed. She should be told at the end of the interview whether her application is approved. Then her passport will be posted back to her by courier service in the next few days.