spotted with another woman

i need to know what to do- almost 2 months ago, my friend spotted my husband with another woman at a coffee shop. At first my friend thought the woman with my husband was his mother, with a new hairdo. but on inspection she realized it was not my mother in law, but a woman in her 40's. My friend said her heart was racing and she didn't know what to do, she was sure it was him. my friend told me about the incident when we saw each other at our jobs the following monday. she asked me if i knew that my husband was at the coffee shop during the day. she asked me if i knew he wasn't alone. i answered no and no, because i had been holiday shopping close to home, and my husband was at home with my children. i don't think my friend would have lied to me about this, and my husband swears it was not him. what would you do? i have never had an incident like this, but our marriage has been hurting for a long time. lacking of affection on a daily basis for years. i am confused and don't know what to do- it is really still on my mind. my friend had discussed it with me after the incident, and after i told my husband. but now avoiding me. he is adamant it wasnt him, and he is definitely mad about this, and mad at my friend. help!!!!!