Without clarifying the way the spell works ahead of time, I think you open up yourself to the possibility for dissatisfaction and a rules debate during what sounds like is an important moment in the adventure or campaign. I suggest doing what you can to head that off at the pass.
Yes, that's the risk. The answer, is likely just to present it as a general rules discussion (which, essentially, it is) nice and early - and see how everyone takes it.
The funny thing is - 7/8 of the group would be just fine with this without discussion (I've been DMing them for a long time and am 100% certain). the 1 that wouldn't be (ok without discussion) is the one that's likely going to be the one with counterspell by the time this comes up.