My husband tried to put in a cement patio last night. Today it is sunken in. Can he lay new cement on top?

He first cut the ground, put in 2x4 and plywood to box it in. Then laid gravel/rocks and some metal grids. He spent about 4 hours laying cement in it, but did not cover it with plastic. Today, it looks like someone stepped in the middle of it. Can he lay more cement on top to layer and even it out? What should he do? (he is not very handy, and neither am I, so "layman's terms" would be appreciated)