If the state she resides in has legal separation then it is okay to date while legally separated - no risk of the other spouse charging adultery.
In my state, that still has many old laws on the books, there is no legal separation - one is married or divorced. Which makes dating while waiting for a divorce to be finalized a little trickery as one spouse can actually charge the other with adultery. Lovely, eh?
From a moral standpoint - which is where I think you are coming from - Why should this bother you? Your friend is free to live her life as she sees fit. As her friend, you should accept her for who she is, and not judge her.
Separated means, according to Dictionary.com - (of a married pair) to stop living together but without getting a divorce.
If they are not living together as husband and wife, and are planning to get a divorce, why can't she date?
So, no, I don't think it is cheating.