Show them the Master Skill List in Chapter 6 "Skills", it's just before the descriptions of each skill. This will give an overview of what stats affect each skill.
BTW, SD is a good stat for lots of professions - it's good for spell casters (every caster uses SD and one other stat for their spells, and PP Dev uses SD + In); good for hit points (Endurance uses Co + SD); good for stealth (Stalk & Hide uses SD + Ag); helps with Perception (SD + In); Martial Arts styles use SD; and more. SD is good choice if the player can't think of anything else to put some bonus points into.
For a Mage, I'd say that Re + SD is more important than Re + In - although RE + SD + In is even better (a human Mage can have +3 in all three and +1 in something else, maybe Qu to help with DB and Initiative)
BTW, stat bonuses don't have a huge effect anyway. The effects of stats are more noticeable at low levels, but are rapidly outpaced by skill ranks (even at level 1, 6 ranks in a skill gives +30 and at level 2, 9 ranks gives +45) and bonuses from Professions and Talents (typically ranging from +10 to +30). Most balanced characters won't have more than a +10 bonus for any stat so at most that will add up to +20 (+10 from each of two relevant stats)....certainly good and worth having, but not worth worrying too much about - the difference between having +5 in a stat and +7 or +8 isn't that big a deal.
e.g. St and Ag are good for weapon skills and most combat styles, but even a clumsy & weak (low Ag & St) character can be a dangerous fighter due to skill ranks (and can improve their physical stats too as they level up).
Also worth noting since you're coming from D&D: +5 in HARP is roughly equivalent to +1 in D&D....but not as significant because it's much more common to get bonuses & penalties of 5, 10, 15 or more to various skills.