Kowalski is the genitive form of a noun
No it's not. Kowalkski is an adjective and it's in the nominative form. Genitive would be Kowalskiego.
Why no change to NOWAK in the female name?
Names are quite complicated. According to the book I have in front of me (Kiedyś wrócisz tu... by Universitas press):
Masculine surnames ending in -ski, cki, dzki and female ending in -ska, cka, dzka decline like adjectives.
Masculine surnames ending in consonants (including foreign surnames) such as Nowak are treated as masculine nouns.
Feminine surnames finishing in a consonant or -o (such as Nowak) do not decline.
Both male and female surnames ending in -a decline like female nouns.
Male surnames ending in -o decline like those in -a.
Female surnames ending in -owa or -ova decline like female adjectives. Those in -ówna like female nouns.