As George says, M is not a taxable event unless the rollover requirement was not met.  So you'd need to check with your client whether there was an indirect rollover, which should be reported on a F.5498.

The window used to be 60 days, just like normal rollover, but TCJA has changed that to the due date of the return, including extensions.  This means your client still has time to cover this and avoid the tax and early withdrawal penalty if it has not yet been rolled over; the F.5498 will then show the late contribution in Boxes 13a, b, and c.

If the rollover is completed within this extended timeframe, you can simply select 1 as the distribution code and enter the same amount under "Indirect Rollovers", like you would with other types of indirect rollover.  Otherwise, you can enter 1 as the distribution code and the amount would be subject to tax plus the early distribution penalty.