Choices:
A) Your client doesn't have bank feeds enabled
B) Your client has Business view selected in their settings
C) Your client has turned off the Banking option in their settings
D) Your client has the Simple Start subscription for QuickBooks Online
Choice B
Explanation:
There are two views when it comes to Quickbooks Online: Business view and Accountant view. Accounting view is for more seasoned users who have some form of accounting training so it has more features.
Business view on the other hand is simplified for those who don't know much about accounting. It is easy to follow and some features are reordered for simplicity.
The Banking tab is one of those features and it is not openly shown in the business view so the most probably the reason your client can't see it is that she is using the Business view.