Amending the plan is not required, just for her to transfer her account to you. She can establish her account with you, and transfer her balance to you, when permitted . She would need to provide you with a copy of the Form 5305-SIMPLE . So, for her account with you, the documents would be a copy of the Form 5305-SIMPLE that was used to establish the plan and your firm's SIMPLE IRA Adoption agreement ( Form 5304-S or Form 5304-SA). All new SIMPLE IRA contributions would need to be made to her old account ( because of the DFI rule under Form 5305-SIMPLE), so that would have to be kept open, and the contributions can then be transferred to the account with you, when permitted ( there might be restrictions on when amounts can be transferred- those, those are hardly ever instituted). Each employee is permitted to do this. The account with you would not be eligible to accept new SIMPLE contributions- only transfers and rollovers.
A SIMPLE can be amended only as of January 1 of a year .The amendment should conform to the 60-day notice. So it might be too late for 2020. If the plan is amended to a Form 5304 , then all the participants would be permitted to choose their own custodian ( No DFI)