The Dad/Haley relationship in the Solo route is fascinating. He obviously suspects what is actually happening between Haley and the MC. He blames the MC completely and he is trying with all his might to put a stop to it.
When the shit hits the fan, Haley goes completely off the rails. Actually, it's not dissimilar to how she reacted when Mom called when Haley was about to do something with the MC in a much earlier episode. It is interesting that she is so ashamed of what she is doing in relation to her parents and especially with her father, whereas the MC does not seem at all ashamed.
Of course the MC knows that Haley is 100% committed to HIMSELF, and was the instigator all along.
At any rate, when Dad is faced with the reality of losing both his son AND his daughter, he caves, which I find eminently reasonable. He suspects, but does he really want to know.