1. Your friend can certainly file a criminal case/complaint of adultery against her paramour ( not against his wife since no adultery case stands against woman probably it is believed that women can not commit adultery,
2. However, the question is whether sending her paramour to jail on the charge of adultery will save his marriage or not,
3. A clear legal advice can be given by asking him to file a criminal case/complaint leveling adultery charge against her paramour but your friend shall have to foresee its affect on his wife,
4. Won't she become furious for sending her paramour to jail?
5. Probably your friend shall have to decide whether he should try to save his marriage at all or not when his wife is seeing some other person.