Ackman is short in the $40s average. A $55 buyout would cost him $300 million. Not good but hardly a severe blow! Icahn wants to see this sold at 15-20x EPS, which would be about $75 $95 a share. Maybe more. Still not a fatal blow to Ackman, but at the higher end of that range would cost him about $1B in losses.
If Icahn made a $55 offer this could trade to $60 or $70 because Ackman couldn't cover his shorts fast enough. There has to be shares available for him to buy. This is the danger to being so deep in a short position in one stock. I would be getting out of his fund if I was in it.