Also, why wouldn't the understudy be dressed to perform?
Like a third string quarterback running onto the field with his headphones and clipboard.
Usually an understudy knows before the opera starts if he/she will be singing the role. It's pretty rare to have an understudy called on in the midst of a performance.
Weird.
Well, if the performance has already started and is underway with the contracted singer, I guess the understanding is that the understudy is not really needed for the night. Usually understudies get called up to action, so to speak, when the first-string singer gets sick or something. This usually happens BEFORE the performance. ;-)
The tenor was apparently healthy. Nothing else than a heart attack could get a normal person to yield his part. Of course, musicians are a tense bunch sometimes.