This is what I would do because whilst it maintains her independence it also gives her the tools to be independent. It also works for anyone of any age whose English may not be so good.
1. Print of the terminal maps from the HAL website and giver her point to point directions as best you can - here is the check-in desk, visa check here then walk here to security etc
2. Explain to her the procedure for check-in and security - especially re liquids and emptying pockets etc etc If she is prepared she is much less likely to get it wrong and cause problems.
3. have some cards / or pages in a note pad with some of the questions she might need to ask - the first page / card being 'I speak little English'. Others might be 'Is this the right way to gate xx?'; or 'How much?' for the shops / cafe. Do something similar for onboard as well e.g for getting a drink
4. Most of the signs are pictograms so she should be ok in finding the toilets.
5. Tell her who to look out for to ask - so BA staff in uniform, the HAL staffers etc who are much more likely to be used to helping people navigate the terminal than some random stranger.