。
She hid the change in her feelings toward him.
Literal
She [topic-は] him [toward-に対する] feelings [genitive-の] change [object-を] hid.
~に対する is the prenominal (modifier) form of ~に対して — '(toward, with respect to) X.' Used here to modify 感情 ('feelings') as 'feelings toward (him).' This is a useful pattern when the 'toward X' relation needs to attach to a noun rather than an action: ~に対する不満 ('dissatisfaction with X'), ~に対する敬意 ('respect for X'), ~に対する反応 ('reaction to X'). The whole noun phrase becomes the object of 隠す ('hide').