Hate to break it to you, but yes, the mother DID force the child into seeing whatever lay behind that door. According to M, she held him by the hand and stood there letting him take it all in- the surprise entrance, and the ensuing reactions, etc. As a parent, I cannot imagine taking my child, at any age, to confront my spouse, argue, fight, whatever. This was a highly charged problem between the parents, and she brought the child along in order to inflict pain and shame on the husband, used him as a pawn with no consideration for how this would effect the child. For the mother, catching them in the act of sexual intercourse was PERFECT for her purposes. She FORCED this on the child.
Edited to say that it occurs to me that some people with very strong opinions do not take the time carefully read what others take the time to thoughtfully write, and tend to miss or distort statements or respond without asking questions for clarification. lol