He is a little snappy in her opinion so she thinks he may have been mistreated and has people trust issues.
I guess I'd be a little snappy and bark if I had been taken out of my home and put in a strange prison.
I hope everything goes well tomorrow. Better to meet off lead and in a spacious but enclosed space if possible IMO. Fern can be snappy (sort of, she screams rather than snaps) if restricted on a lead and a strange dog invades her space. Off lead she is fine as she can take her time to circle and sniff from a distance iyswim. New dog could be similar, or he could be absolutely fine.
Can you walk with new dog on a long line if Rommel is offlead, or is that not allowed?
Take treats by all means, but I am not sure you will be able to detect food aggression at this stage as it is not a normal situation. It might give you an idea of his ability to understand 'sit' and 'gently'
though. Just make sure you treat both equally. Getting them to sit together and treating at the same time might be an idea to begin with.