A few links which might help.
First is a 2008 review. This has the main issue with most of Rushtons work that it uses cranial capacity as a proxy for actual brain size.
https://www.tandfonline.com/doi/full/10.1080/00207450802325843
If your friends are scientifically minded they might question if brain size impacts intelligence at all. This review supports evidence of correlation. https://www.sciencedirect.com/science/article/abs/pii/S014976341500250X
Finally we have more recent MRI based studies by the most based Emil Kirkegaard (from whom I learnt much forbidden knowledge)
If anyone of your friends want to take a high brow scientific approach then hope this helps.
(post is archived)