I just don’t understand his logic..

Our 1yo is having trouble passing stool. We’ve tried prunes, juice, high fiber foods and nothing seems to be working. Called her doctor and they said to give her miralax or lactulose. He wrote a prescription for lactulose but we had some miralax that my MIL left behind. We agreed we’d just give it to her this once so that she can be a little more comfortable until I’d pick up the medicine the next day. I checked the expiration date and it expired April OF LAST YEAR. I told him we won’t be giving it to her. He said “medicine doesn’t expire”. I said yes it does or it wouldn’t have an expiration date. He said “I’d still take it” to which I responded “okay but you’re an adult with a stronger immune system so if this were to make you sick you’d fight it off easier. The baby wouldn’t and it could be dangerous giving her EXPIRED miralax” he walked away all irritated and mumbled “I would’ve still gave it to her”. I’m so irritated. I just needed to vent.