Well, 4 star shields lvl 10 heat are nearly as good as 5 stars level 10 heat. 5 star shields only surpass 4 star shields in stratum 6 and even at d28 the difference is small. The health bar is longer for 5 stars but the percentage of shield health lost from a hit is the same. Darkfang also fully negates the shock and fire status, just like grey owlite shield. The difference between them is that darkfang shield exchanges some elemental defense for normal defense. It's not going to be a big difference here either, but in general I'd favor special defense over normal defense.

Basically an Elemental + Fire/Shock shield is very useful and you have a few options, I think I'd rank them something like this, from worst to best:

Grey Owlite Shield lvl 1~7(?) < Darkfang Shield lvl 10 ≤ Wise Owlite Shield lvl 10 ≤ Grey Owlite Shield lvl 10

But the difference is small and you won't notice it unless you really pay attention. It's more important to get many different shields to cover as many Damage+Status combinations as possible. E.g. You'll also want Elemental+Freeze and Elemental+Poison shields. So if your friend has a darkfang shield, they now have a great elemental+fire/shock shield and should focus on getting other shields like dread skelly, crest of almire, dragon scale shield, royal jelly shield and heater shield that they are still missing.