When favorable is followed by a preposition, the preposition is usually either to or for. When the meaning of favorable is "feeling or expressing support or approval," to is used:
... stengthened the hand of those favorable to the council —Paul Sigmund, Change, March 1973
... because all men are favorable to that process — Walter Prescott Webb, The Great Frontier, 1952
Several... generals were mentioned as favorable to the scheme —American Guide Series: Texas, 1940
For is the usual when favorable means "suitable":
... the university structure is more favorable for accurate evaluation of societal problems —Halton Arp, AAUP Bulletin, September 1969
... to select those most favorable for hybridization —Current Biography, January 1964
Both to and for are used when favorable means "advantageous." To is more common:
... constructive acts favorable to the status of the Negro —James B. Conant, Slums and Suburbs, 1961
Existing credit law, sometimes archaic and traditionally favorable to the lender —Lucia Mouat, The Consumer Fights Back, 1970
... the reform was favorable for wage earners but the opposite for pensioners —Current Biography 1950
An alternative preposition after favorable is toward, which is used from time to time in the same way as to:
... reviewers were not too favorable toward the production —Current Biography 1953
(资料出处：韦伯斯特英语用法词典)