"Also, 1099-nec for 2025 will likely be received in 2026."

The tax year is on the top of the form. It is for 2025, it is sent in 2026 in time for the 2025 tax return. But your client should have their own business records. If your client also has non-business customers, they would never need to send a 1099-NEC. That's why your client should have all their own records.

Your client is likely a Cash Basis tax payer. Even if they sent out bills for the work, the payments are reported based on the date they get the money. If they get a check in December and don't take their bank deposit until Jan, that is still Dec money, because the banking is not when they got paid.

Accrual basis is reporting on things known, even if they haven't been fulfilled. Performed/earned is Accrual. Look at the prior year tax form, it will indicate if they have been reporting on accrual basis, but that is highly unlikely.