Dear Tony,
I'm sorry I can't shed any light on what 'U-boat' this could have been but have you considered the possibility that it could have been a friendly submarine just 'keeping an eye on things' or in this case a periscope.
Deck crew on ships tended to suffer from the well known disease 'periscopeitis' and everything they saw in the sea was a periscope and a U-boat periscope to boot.
I'm sure that if it was a U-boat and its commander thought the "Queen Wilhelmina" was a worthwhile target it would most certainly have attacked your mothers ship. U-boats when they ran out of torpedoes (not often) used to get home to base or resupply as soon as possible. Also at that time it would have been armed with a deck gun which U-boats (many) preferred to use if possible to conserve torpedoes. Maybe 'Prize Rules' had an influence on the commanders thoughts?
There are people on this forum who would be better equipped to answer your question than me, but you have my thoughts on the matter anyway...
Yours
Harvey