I have a few house that are doing okay, but when a leaf falls off the plant I leave it in the container on top of the soil. I feel that as the leaf decomposes it helps the plant itself. A friend say NO. Never leave it on the soil but put it into my outdoor compost pile.
Will it harm my indoor plants by leaving the leaves on there?

It's best to keep the soil cleaned up, something I do every week while watering. One leaf or so shouldn't be a disaster, but when you get a lot of dabree in the pot, it can then become a great place for pests to hide, not to mention a great place for mold to grow. Some parts of the plant that falls could even be diseased. It's safer to keep it as neat as you can.