If you want to keep the lid on a bio cube you can use purigen which is a chemical skimmer. You simply replace it when it changes color which is a few months. I'm not a fan of nano mechanical skimmers since it's a challenge to get them to work well and they don't fit under the stock lid.
I had higher levels of nitrates in the tank and have since added a refugium. I simply used egg crate to box out the bottom chamber of the stock media basket, I have a grow light attached to the outside and taped the rest of the second chamber so I don't have the glow at night. My nitrates are now 1ppm. They use to be much higher, over 25ppm. I take out half of the chaeto each water change (bi weekly). That's another option to get the nitrates down.
How often is you friend performing water changes and how much? Are they over feeding? DO they vacuum the sand bed during those water changes?
@Salty.Reefer