None of the above. Tell him that people are spreading rumors about her cheating on him and you can't believe them, but he should tell her about the rumors. Because she would never do anything like that. Tell him exactly what your friend told you, but there's no way sweet little Brunehilda would ever do this.
Then back it up with more evidence, like "I've even seen her out with another guy, but I knew it was just platonic, because she's not that type of girl".

Now your friend has all the info, you're on both his and her side and if he decides that it is true, you can then join him in the surprise and anger of being duped by sweet little Gundleglitch.