The Volendam certainly is not a small ship - she's 63,000 tons and that's not small.
Having been on her a number of times and going through some rough weather, I can tell you that she rides well. If your friend is worried about seasickness, she can take Bonine (Bonamine in Canada) or other seasick medication. I find that Bonine works well for me - I start to take it a day or two before I sail and then usually take 1 a day.
Another thing is to book a cabin low and in the middle - Main or Dolphin decks - not as much motion down there.
Hope you succeed in talking your friend into cruising with you. You will love the Volendam.
The sea bands work well and if you don't want to take chemicals you can take capsules of ginger or drink ginger tea. I take both with me but have never had to use anything but the bands and we have been in weather that produced 20 foot waves - also very rough from San Diego to Hawaii. These ships handle the weather very well and I believe that the stabilizers help prevent sea sickness. by the way, the ship was the Statendam and it is smaller than the Volendam.