The Lord said unto Moses. Presumably in the tabernacle, whither Moses would have returned to supplicate God. If her father had but spit in her face. The "but" is superfluous, and obscures the sense; the act mentioned is referred to not as something trifling, but as something in its way very serious.
The Septuagint renders it correctly εἰ ὁ πατὴρ … πτύων ἐνέπτυσεν. The Targums have, "if her father had corrected her." Probably they used this euphemism from a sense of a certain want of dignity and propriety in the original expression, considered as coming from the mouth of God.
The act in question was, however, not uncommon in itself, and in significance clearly marked (see Deuteronomy 25:9). It was the distinctive note of public disgrace inflicted by one who had a right to inflict it.
In the case of a father, it meant that he was thoroughly ashamed of his child, and judged it best (which would be only in extreme cases) to put his child to shame before all the world. So public a disgrace would certainly be felt in patriarchal times as a most severe calamity, and entailed by ordinary custom (as we learn here) retirement and mourning for seven days at least.
How much more, when her heavenly Father had been driven to inflict a public disgrace upon her for perverse behavior, should the shame and the sorrow not be lightly put away,, but patiently endured for a decent period!
(cf. Hebrews 12:9).