I have seen the moon is daylight before but that isn't any proof of a flat earth.
The moon can be seen in daylight sometimes, but the article that mike tesko linked to clearly states "At full moon, the moon is exactly opposite the sun", which implies that if you see the approximately full moon in daylight, the sun isn't in the same area of the sky, but instead about as far away in the sky as is possible - both will be on or near the horizon, but in approximately opposite directions from the observer's point of view.
It also means that when the moon is very close to being a full moon and is wholly above the horizon, the sun is below the horizon (in the approximately opposite direction) and so not directly visible at the same time as the moon. This situation can occur shortly before sunrise or shortly after sunset.