I think the mother might have been lying, but at the same time maybe the girl is lying to her mom, saying she went out to play tennis, when she actually is doing all of that sex, and the mom knows this because she read the daughter's text messages or whatever, hence why the daughter asked the mom whether she read them. So playing doubles tennis (a game with four players) is her alibi for having a foursome with three older men all day, hence why she might be tired when she gets back. But on the other hand I think it is meant to be deliberately ambiguous whether the mom was lying or not -- she definitely cucked her daughter, it's just ambiguous if the daughter was already cucking the dude to start with. I could see both being true.