I saw my son-in-law kissing another woman?

I have a secret My son-in-law works in chicago and commutes daily. About a week ago I was in chicago visiting an Art Gallery with my friend, and when we went to catch the coach back to our home town I got the shock of my life when I saw my new son-in-law at the coach station, obviously waiting to catch the same coach as us, kissing and cuddling another woman! I was routed to the spot and hid out of view where I watched him board the coach. The woman waved her off and was blowing kisses as the coach pulled away. he didnt see me. I have suspected for some time that he was having an affair. The person he was kissing is definitely not a relative and you dont kiss and cuddle friends in that manner. I think it must be someone he works with. My daughter would be devastated if she knew. I wish now I had had the courage to walk up to him when he was with this woman, but I was in shock. My daughter trusts this man implicitly but I have always thought he was a liar. Can anyone suggest anything? I have no proof of the affair, apart from what I saw that day.