Use of 'formidable'

Question

I need help understanding this sentence that one person says to another: C'est formidable que vous avez l'air seul.

"It's great that you look alone/lonely" just doesn't make sense, or, at least, it's not a very nice thing to say to someone.

Alan Graham · Accepted Answer

This is not really a forum for requesting translations, but I was intrigued so I googled it to find the context. I think it's from Sartre's Age Of Reason, and you've slightly misquoted it, it should be:
C'est formidable ce que vous avez l'air seul.
The structure "c'est ... ce que" makes formidable function as an adverb applied to "vous avez l'air seul." 
In this context formidable does not have a positive sense, it means "terrible".
So the whole sentence means "It's terrible how alone you look" or, simply, "You look so terribly alone."