the rhubarb should be outdoors planted out, rather than being mollycoddled inside. it won't mind the frost at all. i would put the poorly one out too. it'll soon recover hopefully.

New leaves can get a bit frizzled in a sharp frost but normally recover well. If your greenhouse is warm it might be as well to harden the rhubarb off during the day for a week or so, to avoid the shock of a sudden change of temperature. This is because in a warm environment the stalks will be less sturdy and so liable to frost damage, whereas ones that have been outside all the time are tougher.