There are a lot of rude words, of course, AVim. She would become his mistress, in the language of forty years ago, and his new partner in today's language, unless the relationship was very transitory and she found she'd made a terrible mistake.
"Mistress" is a rather old-fashioned word these days although it is still used. It means the girlfriend of a married man. It does not necessarily mean that she stole him from his current partner. Suppose a married man has been seperated from his wife for 2 years but is not yet divorced from her. He found his new girlfriend a year ago (she did not "steal" him from his wife). Technically, because he is still married, the girlfriend is his "mistress".