Well if the breakup did happen, and it happened on Thanksgiving, Kelsey's mom might not have been able to find out about it, at last least directly from Kelsey. She went missing that day, and since she talked to her mom in the morning but was seen at Safeway in the early afternoon, she could have already be missing. Depending on time of day the breakup occurred, if it occurred during the child pick-up, or over the phone beforehand.
Another thought, if they did breakup and did so over the phone before the daughter was picked up/dropped off, PF may have already been upset and volatile when he next saw Kelsey. He may have had time to plan. jmo