Depends a lot on what they trade and if they make markets or take risk but generally i would say
Practice mental math on arithmetic.zetamac.com. aim to get above 60 consistently in 2mins. tradermaths.com/tradermath is also good so you are familar with decimals and negatives.
Practice leetcode
Practice brainteasers (look up past questions on glassdoor for different companies)
Practice probability questions e.g. bayes theorem, combinatorics, expected value - again on glassdoor or interview books
If they trade options be familiar with basic option strategies (spreads, flys etc), option pricing, pay-off diagrams, and the greeks