Just raise and take down the sitter, "most" of the time the other player(s) will notice this and so on.
One time there was three of us left and the big stack decided to "sit out" me and this other player had smaller stacks. Anyyyyway, we was playing the sitter more than outselves and took his stack. This might sound lame, but I hate people who sit out waiting for others to stack off out and get headsup. The guy whined and came back in saying we was colluding...However this wasn't the case the guy thought we would bust each other and he had his guranteed 2nd place.
We never once typed anything to each other, but both looked at it logically, free chips from a big stack sitter. I'd of sooner got headsup with a guy willing to play than some idiot waiting for someone to bust.
Anyway he ended up going out in third place after all and said he was emailing support. We did nothing wrong.
Of course there are times when a short stacked player decides to sit out and you're mid/chip leader against another bigger stack and they don't understand the logic in getting the sitter to bust first so was raising me. When this happened I'd either fold or push.