Your helper is definitely entitled to open a bank account in Hong Kong and we strongly advise that she does so. However, some banks are not very welcoming to helpers; either they might face discrimination from front-line staff, or they might be asked for many supplementary documents which they do not always have or know how to access. Some of these documents might be easy for you to provide (e.g. proof of address), but your helper might not know whether she can ask you for this.
For this reason, many employers find it helpful to accompany their helper to open a bank account. The account will not be in your name and will be entirely hers, but your presence can make the process smoother and ensure that she can feel more comfortable to ask you for any necessary documentation. We also advise paying her salary by bank transfer where possible, as this is a good record both for you and her, and will also help her improve her savings, rather than having cash always available on hand. You can even encourage your helper to open a second bank account and have an automatic transfer from her main bank account (where her salary is transferred to) to this second one for savings.
Although we do not advise any particular bank, you can check out our handy comparison table of different banks and their requirements.