Just tell them something like:
"While you were out of town, a thought struck me. I know you've been concerned about his naps and I wanted to try to do something about it for you, so (this) is what I tried. He actually responded really well. I guess some kids just need a lot of exercise to wear themselves out for naps. Maybe if he's being more active in the mornings on a regular basis he'll take his naps a little better. He's your son of course, so what do you think? Maybe it's worth a shot?"
That being said, I'm not sure why they are insisting that a 13mo still take two naps per day. That just seems a little excessive to me, but then I have no idea how long he sleeps for at night. With babies, it's less about how many naps and more about how many total hours of sleep per 24 hour period. Both of my sons were down to one long nap per day by the age of 9 months.