She was telling him to do a one-time chore. The lecture was listing all the bad things that would happen if he didn't do the chore (that he had already agreed to do).
MIL is apparently speaking to us again. She yelled at DS on the phone at the time, then the next time DH spoke to her nearly a week later she yelled at DH about DS's behavior, then she did her standard 'almost-silent-treatment-until-they-figure-out-I'm-upset-and-grovel.' Needless to say, we did not grovel. So I don't know if she let it go, or if she's clinging to a vague memory of DS being horribly rude and disrespectful. Either way, it will be a long time before he talks on the phone to her again.