Sustaining Focus and Concentration Options

[shadowbod]

When a mage is sustaining a spell, the GM has the option of making them roll to keep sustaining when their concentration is broken, e.g. when they take damage (see step 7, p174 of the rulebook).

Do you think that the mage's player should still have to roll if they have had the mage cast the sustained spell through a sustaining focus?

The reason I ask is that there is a mage in the group I run and he likes to cast invisibility through a sustaining focus. I know that there are lots of ways around invis and also that the spell could be dispelled by another mage (or whatever) etc. I liked the fact that if he was sustaining the spell himself and was unlucky enough to get seen and shot, the spell might fail, but this doesn't appear to be the case when casting through a sus-focus as it is doing the work for him.

My preferred option at the moment is to say that although the sus-focus takes away the need to concentrate on the spell to sustain it, it is still the mage that is actually responsible for keeping it going and if he gets shot, he will have to roll for concentration.

What do you all think?

-edited for clarity-