I would not remove the cowl tag for any reason and in fact I taped it off to keep the original Janesville paint scheme around it on mine BUT..
Since he has the build sheet I think he is OK. I would be satisfied that it is legit by matching up the hidden partial VIN on the cowl with the build sheet and the VIN on the door (I'm assuming that was not removed.
The cowl tag data can be verified by the build sheet.
Again I think it is a bad practice to remove the cowl tag but I think he is covered on this one (once verified).