Sleeping With his Mother at 31

Its about my husband, I go away on holiday at times with a girl fried for a week, while I me away his mother comes to stay she is good cook and will spoil him, after about four days on holiday my friend got sick so we came home when I got home nobody was in, I unpacked an took my case in the spare room where his mother was sleeping I noticed the bed was made as I had left it then I realised the bed hadn't been slept in, I had put fresh bedding on before I went away, I thought no he is to old to sleep with his mother, then a wicked thought crossed my mind I thought no that's disgusting to think such a thing as i****t, I examined our double bed there were stains I know what the stains were from, when his mother came home she was surprised to seem me, and went straight up stairs to her room, later I found her bed clothes ruffled up, am I the only person to discover this relationship I think I am, I've herd about it but didn't think it could be happening between mother and son, or am I in denial, am I the only one?, how long has it been going on, he has always been a mums boy, answers please