My general rule of thumb about how to treat others is to treat them as you would want to be treated. In other words, if you were not sure if you would tell her, ask yourself if you were in the same situation would you want to know.
If so then you should tell her and you owe her that service. If you think you would rather not hear that from the cards then don't impose that on others. There are always tactful ways to tell people anything if that is what you want to do and every story always has two sides which can both be presented.
I can't speak for you but I can only tell you my feelings. IF if were me, yes it would be hard to hear but I would want to know. I would need to know. Because if I did not know I would be getting closer and closer to a man who could not be trusted. Suppose we DID get married and he cheated on me then. Things could get much worse than they are now.
Also, on the chance that the problem could be fixed. If I knew now and I confronted him with it, there is potentially still the chance that things could be fixed still with councelling of some sort, but the sooner that is done the better. It one waits too long it could be too late.
Not to mention, the worst case scenario in which he gets some STD from another woman that your friend's daugther could catch.
I can understand that it's no fun to tell anyone that kind of thing and you don't want to hurt them, but it is possible that they could be more hurt by it in the future than if they new and cut things or or were able to fix them now. And if you do decide to tell her, make sure. I would do an additional reading on my own to make sure of my facts and ask the cards what is the best way to tell her that would be most helpful and inflict the least amount of pain.
And absolutely you should not give advice afterwards on what she should do, just to tell her what the cards say.
Since she asked for the reading, it does not make you responsible for what happens afterwards. She set the ball in motion when she purposely asked you for a reading on the topic. She sought out info on the topic and you just gave her what she asked for and needed. If she was intent on knowing, if you had not provided her the answers, she would have sought them elsewhere.
Just my thoughts.
Babs