It should be taken into consideration that Freud's theories weren't very scientific nor falsifiable (although this is sometimes debated); my point being that, given a case, we might be able to use Freudian theory both to accept or reject the application of cultural relativism in that case. However, Freud did not generally attribute much truth to the idea.
The concept of relativism is comparable to the maxim "beauty is in the eye of the beholder" - that is, subjective concepts are given value by subjective observations. Cultural relativism is sometimes distinguished from moral relativism, but the two generally refer to the same area of thought; that patterns of behavior and the values we ascribe to those behaviors are subjectively determined by the societies we live in. Infanticide was right for Spartans, but wrong for suburban Americans, and so forth. Very broadly speaking, this implies that all humans are a blank slate when we are born, and the imprinting of our societies is what determines our value systems.
Freud was more interested in a universal notion of human nature, particularly because dreams, instincts and repressions seemed to imply that there was definitely some kind of fundamental drive in human nature that asserted itself regardless of one's specific upbringing. For example, Freud considered the Oedipal complex to be an innate, instinctive, unavoidable and indeed normal part of sexual and identity development.
On the other hand, Freud felt that a large part of mental illness/neuroses, etc. were the byproduct of society repression of sexual instincts. In this way we might argue that the cultural relativism related to "right" or "wrong" sexuality can have an effect on the psyche, however it seems as though Freud did not consider it possible for a repressive society to have anything other than a detrimental effect. For Freud, the purpose of civilization and society (and culture) were the repression and redirection of certain instinctive drives (such as destruction and sexuality) and thus the relative values found between societies are far less important than their similarities.