My friend told me that she caught her
If you want to support your friend, then you need to relate to her and not intercede in their marriage directly by talking to her husband.
I'd be serving divorce papers... To do any less is to invite yourself to be cheated on indefinitely. If you want to be a good friend, be supportive of how she feels.
Welcome to the board, Workingmomng ~
First, I have to say while I do understand you are concerned for your friend and her marriage, it's not your place to contact your friend's husband to discuss their issue with him.