Work performed physically in Paradise for a Canadian employer is not U.S. employment, so the current arrangement can continue. Work performed in the United States, even briefly, requires status, and the resignation should wait for the admission.
The border, not the employer, marks the start of U.S. work
The developer's present situation is lawful: she is employed by a Canadian company, works from her home in Paradise, and the U.S. client is her employer's customer. Nothing in that arrangement engages U.S.
immigration law. The status question begins when she performs work while physically in the United States, whether for the client directly or for her current employer during a visit. Once the direct offer is accepted, the sequence is: retitle and finalize the letter, gather the diploma and experience letters, choose between a port-of-entry application and an advance petition, obtain the admission or approval, and only then resign and move.
Attending meetings at the client's office as a visitor before the TN exists is limited to activities visitors may perform, and hands-on work on the client's systems during such a visit is not among them. If the client wants her in the office by a fixed date, the file should be prepared to meet it, but the date should not pull the resignation ahead of the admission.