Should've dragged that bike out before ever standing it up. Quicksand 101. Do not stand in it, if you get stuck, lay out and crawl, same for your bike... Your buddy had the right idea.
Had that happen once with a Friends 400ex, And is the reason i tend to avoid wet areas on the Dunes. Thank god some guy in his Sand Rail came by only way we could get the thing out. Awesome ride other then that.
Yep, done the exact same thing on my kx500. If you notice soon enough you can usually sit way back and power through but it's scary stuff. It's usually a combination of wet plus a depositing area below a little dune droping sand below onto the wet. I think as the sand is deposited when wet it bridges air and get super soft. Mostly a problem for bikes but I've seen it swallow side by sides a couple of times now too. seeing water standing isn't the only place, just has to be really wet sand that piles. It is mostly just a problem in early spring when the sand is wet and blowing/drifting more regularly but it can sneak up on you even after knowing what to look for. Best to stay far back on the seat and quick to grab throttle at short notice.
I was out riding in OR. I saw a quad in the distance and rode up to it. No-one was around but nearby I saw the top of a helmet with a goPro on it so I grabbed the GoPro and went home. Once I watched the footage I immediately went back! and grabbed the quad....