My wife is 5'8, has a Aire Lynx, and uses a Werner Rio 220cm two-piece paddle and likes it a lot. Looks like the Rascal is narrower in width by a few inches so perhaps 220 is too long...

I'll echo Brian and Fish's sentiments. Traditional "wisdom" has always been to get long paddles for IKs.... 220s, even 230s. But those of us who have paddled them a lot over the years have gone to shorter and shorter lengths. You don't need a big damn stick to reach over the tubes. I would go with around 200 cm at most. That's a fairly narrow IK too, so even 194 would probably suffice.

You give up a little power not having a long paddle, but you by far make up for it with quicker stroke rate, more finesse, and less shoulder problems. Also you can then just get a standard WW paddle off the shelf without having to get a special long length ordered or get a sea kayak paddle.

Oh, and make sure they get thigh straps for those boats!