If you find out that your girlfriend's husband is cheating, it is best to let her find out for herself and handle her own business.  Friendship is between you and your girlfriend and should not involve her personal relationship with her husband.  Simply speaking, you are not married to your friend's husband and since she is the cheating husband's wife, the problem is totally hers. 

You must realize that although you are not in the household of your girlfriend, she has some knowledge of what is going on in her marriage.  She may have even talked to you about the problems that she is having.  Relationship problems do not grow without a seed and somewhere in the relationship one of them is not pleased.  You are not part of the relationship between your girlfriend and her husband so you need to take care of your matters and leave the girlfriend to handle her own.

You may think that you are the rescuer and is saving your girlfriend from a surprise heartache.  But guess what, your girlfriend is not clueless with what is going on in her  relationship with her husband, you are.  Even if your girlfriend aks for your advice, it is not ethical or good manners  to say that you think that her husband is cheating.  For one reason, you do not have all of the facts.  Since you do not have all of the facts but the girlfriend do , then it is better to let her handle her own affairs about her marriage.Even if you were the maid of honor at their wedding, you still have no right to offer your advice on a wife and husband relationship that does not include you. 

Offering any information about your girlfriend's husband infidelity may have you without a girlfriend because she may think that you are trying to get her husband for yourself.  In situations where there is knowledge of a cheating husband, most of the time the woman who is cheated on is insecure in  herself or lacking something that is important to the relationship.  So the husband tries to make up for it by looking outside of the marriage.  If you were to tell your girlfriend that he is cheating, her insecurities may cause her to accuse you of wanting them to breakup so that you can get her husband.  You have to be very careful when you talk about something that belongs to other people and that include husbands.  The girlfriend may not tell you that she suspects that you are after her husband, but she may began to think of you as the enemy and your friendship may suffer.

Keep the friendship between your girlfriend strong by letting her solve her own marital problems.  Your girlfriend may be well aware of the infidelity and may be looking to get a divorce or to fix the relationship in other ways.  Your innocent contribution could have dire consequences to the relationship between your girlfriend and you when all you wanted to do was to help.  MYOB - Mind your own business on this one when it comes to telling your girlfriend about her cheating husband.  This is one way of keeping a good friend and giving her the props for being strong enough without your help to take care of her own marital problems.