I can see this playing out out 2 ways for you. One way is to stop all communication with her and stop seeing her. Bros definitely before hoes. Now ff she contacts you, be as vague as possible in your discussion and really listen to what she is saying. At some point, you might simply have to ask her. If she tells you that your friend is not her husband, then bring up your reasons why you think she is. If she does confirm that your friend is her husband, be direct and ask her if he knows what she does on the side. If he doesn't know, tell her you can't see her. If she says he does know, then you have a decision to make. Take her word for it and see her, or you could ask her if it is okay to mention to your friend that you know what his wife does. If she gets all defensive, you know she isn't telling the truth about him knowing. At that point, I would tell her good-bye.
The second way you could play this out is to simply allow curiosity to get the best of you. Go ahead and book a date with her, but don't let her know it is not a play date. When she arrives, confront her with you suspicions and then play things out as described above.
There is a third way to find out. Keep your date and bang her as if you didn't have any suspicion. Then after the fact, congratulate your friend on his recent marriage. If he is any type of real man, he will show you a picture of his blushing bride. If not, casually ask if you can see a picture. When you see the picture, you will know right then and there if the girl is his wife. If she is, your call on what to do next. Another thought is to first congratulate your friend, see a picture of his bride, and keep your date with the provider. When you meet her and she is his wife, your call on what to do. If she is not his wife, have fun fucking her brains out
One last question, if this guy is a close friend, wouldn't you have known about his upcoming marriage or at least met his fiancee? Sounds to me like he is more of an acquaintance than a friend. If this is the case, why concerns yourself if this chick is married to him?