I'm no expert, but I know that's a really bad idea. As the mulch decomposes, the patio will settle and be uneven. It will also likely heave from frost. Everything I've ever read says to lay it on sand or gravel too.

Have him talk to whoever you got the flagstone from. He's not going to be happy when he has to redo the whole thing. It's much better to do it right the first time.

Ditto what jugglerguy said.

jugglerguy is correct. For a patio that will last for years to come, install a 4" layer of 21a or crushed stone as a base. then screed 1" of concrete sand as your bedding. set the flagstones in this and sweep sand in the joints.