According to the Mosaic Law, an adulterer and adulteress deserved death (Lev. 20:10, Deut. 22:22), but no man could be put to death without the requirement of truthful testimony of two or three witnesses, whatsoever the sin be, and the hands of the witnesses were first upon him to kill him (Deut. 17:6-7, 19:15, cf. Jn. 8:7)
Regarding the adulteress in Jn. 8:3-11, she was already accused of adultery by multiple witnesses, thus not brought in for deliberation, rather condemnation of her crime, or sin. The adulterer’s lack of presence in that moment did not nullify the Law on adultery for either of them, for reasons explained above.