The book proposes to leave the duration rules out of veils and just make them last a scene, which I tend to agree with, if they are caste preemptively. As a block spell during a conflict, that's a different story.
Potions do not benefit from frequency specializations (they do from power specializations, though). One of their key drawbacks is the fact, that they are always "one use"-enchanted items. If he wants to benefit from frequency boni, he should create it as an enchanted item, which in the case of an umbrella seems a better choice altogether. I kind of have a british gentleman wizard in my head right now.
Potions can be adjusted by declaring aspects on the fly, so he could increase the effect of the potion. Also, it can be given to any other person to use. On the other hand, enchanted items can be charged with multiple uses.