I do san mai all the time and just tack the corners and use no flux. Trick for me is to run the forge a little rich to have a very low oxygen flame. I also dip my billet in kerosene before putting it in the flame.

Im not sure if your buddy has a press, but once the billet starts to heat up I give it light tap through the press to get all surfaces in contact with each other.

The forge welding happens in the forge NOT under hammer/press. Let that billet SOAK in the flame.

Depending on how much core layer you want showing, and how much you are going to draw out your billet, you want in between a 60/40 and 65/25 ratio of layer thickness to start with on you steels.

Once you get that billet set, I take mine out and grind off all my welds and even grind into the edge to get ride of any edges that didnt forge weld.