It's great that you want to help the Sri Lanka worker to become literate in English. There are two questions to ask first:

1. Does she speak some basic English already? If YES, the better. If NO, teach her some Oral English first.
2. English is well-known for an orthography that does not systematically relate sound to a (Latin) character, compare the words "but" and "butcher", for example. You would therefore need to teach her entire words rather than single letters as a work-around until she understands that principle that is so unique to written English.

Unlike you, I think you should start with phonics materials for children; it's not "childish". It may support her self-confidence and may motivate her - what children can learn, she can learn as an adult the better! Give it try and see how she responds.

Good luck!