Typical concrete is a water based mixture (where the term "hydraulic" comes from. (I hate it when the sun hits my monitor!)
As far as psi, unless he is building a commercial structure, his biggest concern for concrete strength is a term called "slump" Any professional concrete supplier will know what that is, and never settle for a slump of less than 7". "Slump" is defined as a test where concrete is put into a 1' tall cone, set on a board, and when the cone is removed, how tall is the pile? If it is less than 7", then there is too much WATER in the mix. You see, water is A LOT less expensive than the other ingredients, and it makes it a lot easier to pump the concrete if there is a lot of water in it. The problem is, a high water content results in a very dark colored concrete that is very very weak. A slump pile of 7" or taller is okay, anything less has too much water in it.
In either case, I wouldn't worry about it in this application. For a driveway, where my 6,000 pound suburban will punch through a spec home driveway, I think differently. (spec home driveway pourers love to "crown" the soil beneath the driveway, so the slab ends up 4" thick on the edges, but less than 2" in the middle, at a considerable cost savings in material, but ends up with a driveway that SUCKS!)