Sandyrivers I goggled fleas in winter in canada and there are some interesting links that indicate fleas can survive our winters. After reading some of the links I would say "yes" the dog could pick up the fleas from the outside BUT he would have had them for awhile now if they have survived the winter. They were likely already in his home and the heat being turned on keeps them alive.
Having said that do you think your neighbour is just looking for an excuse to give you grief about having multiple kitties?
Here is one link but there were lots...
Tabitha April 10, 1995 - August 23, 2013
Bomber April 10, 1995 - July 12, 2010
Winston Nov 15, 1999 - September 15, 2011
Sophie Aug 30, 2011
"UNTIL ONE HAS LOVED AN ANIMAL, PART OF THEIR SOUL REMAINS UNAWAKENED"
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.