You should consult a dermatologist about your wife's skin problem. There are other things to consider other than the freckles, like the moisture of the skin, sun exposure, age, whether she is pregnant or not, previous medications, allergies, etc. Tretinoin is a good medicine for sun damaged-skin, only a pea-size of the cream is required to be applied every night but be aware that it is very sensitive to light, that instead of whitening the skin, it may make the skin produce more oils and melanin that rather darkens the skin.