That is it in a nutsehll. If he turns her away or makes a big deal out of it, he plays into the hands of his detractors.
I'm going to agree with you here. If he was inviting her to his own house it would be improper, but she is invited to the White House...which belongs to all of us...it's a public place.
We are all commanded to be witnesses and to share the Gospel...Dubya included. This is a perfect opportunity for him to share the Word.