She can keep doing her Canadian job from Canada, including work that benefits the U.S. subsidiary. She cannot perform that work while physically in the United States until she is admitted in L-1B status, and business-visitor entry does not cover productive development work.
Location on each date is what matters
The distinction that governs the timetable is where she is sitting when she writes the code, not which customer benefits from it. Working from South Frontenac for the Canadian employer on a project destined for U.S. customers is Canadian employment and needs no U.S.
authorization. Sitting in the subsidiary's office, or in a customer's barn in Wisconsin, doing the same work is U.S. employment and requires L-1B admission first.
A short trip to meet the subsidiary's staff or attend a planning meeting can be a business visit, but installing software or debugging on site is not. Build the project plan so that the pending period is spent on work that can be done from Canada, file early enough that admission precedes the on-site phase, and consider premium processing if a customer commitment is fixed. If the year-abroad calculation is close to the edge, file sooner rather than later, because the qualifying year is measured backward from the filing date and a long delay can push part of it outside the three-year window.