I don't think its a question of either playing outside on concrete with trucks or staying indoors unless you have only concrete to play on and trucks to play with. That's a slightly extreme response to what I was saying which I didn't imply you should do.
I really wouldn't worry about it anymore. There are alternative ways to play outdoors and reduce the risk of wearing the knees out of a child's pants, but I wouldn't entertain this parent and I don't think it is so much a big issue just that she wasn't clear on what her concerns are so she has left you to think at your own will about what she said. An hour a day on knees, on concrete isn't going to damage the pants as you said.