I used to own a Black Palm longbow. It was before I knew as much as I do now about bows, so consequently I broke it.
It was 74" long & roughly a flattened 'D' shape in cross section. It was a smooth taper from the center down to bot tips. The tips were cylindrical & the string held in place with hemp knotted around the limb tip, forming a crude nock. I'd have to guess that mine drew over 80# at 28"
I've done a little basic research into palmwood bows (Since I destroyed mine with ignorance!) They demand a very low brace-height, 3-4" is about right. Even at extrem length like mine was, drawlength is very limited. With your friends 68" I really, really wouldn't draw it much past 20"
The fibers in palmwood don't run for very long, certanly not the length of the bow. 8-10" is more normal. The black, cylindrical fibres themselves are very tough indeed, but the softer, light brown pith is very soft.
Palm bows are generally built by jungle tribes for hunting, mainly, small game, monkeys & birds etc. In the research I did, it seems as though most of these South East Asian tribes are small statured, often no taller than 5'6" & rather than the draw syle we are used to, hold the bow at angles anywhere from vertical to horizontal & draw to their chest with either a pinch or 2 fingered draw.