。
Apparently cheesecake as a peace offering is the way to smooth things over.
Literal
Waves-wind [obj-を] calm [for-には] cheesecake [gen-の] souvenir [subject-が] good seems [よ].
波風を鎮める is a set idiom: 波風 ('waves and wind') figuratively means 'turmoil, conflict, trouble between people,' and 鎮める ('to calm, to pacify') is the formal verb. The idiom means 'to smooth things over, to calm a conflict.' お土産 ('souvenir / gift') here is used in its sense of a peace offering — something you bring to placate someone. ~には (purpose) + が良い (standard 'is good for') forms the 'for X, Y is good.' らしい is 'seems, apparently' (hearsay-ish).