Else Falk’s recollection was that Frtiz was killed by the Nazis in a death camp.
109  The Yad Vashem contains entries for Fritz S Loewenberg, born 19/8/1901 died Auschwitz as below.  It therefore appears that Fritz fled Berlin for the Netherlands.  However this strategy of seeking safe haven failed and he was deported from the Netherlands back to Berlin and then on to Auschwitz.  He was declared dead and a Holocaust victim at the end of the war.
The record of the Digital Monument to the Jewish Community  in the Netherlands 
http://www.joodsmonument.nl/person.php?thg_id=1007.23031&lang=en confirms that the Fritz Siegfried Loewenberg who was born in Charlottenburg on 19 August 1901 and was a Chemist, reached the age of 42, was married to  Maria Loewenberg-Blumenthal.  She was born in Kowno on Kowno, 28th September 1902 and died in  Auschwitz, 7th September 1942. In February 1941 the two of them were living at De Lairessestraat 147 huis, Amsterdam.
Loewenberg Fritz 
Fritz Loewenberg was born in 1901. Prior to WWII he lived in Berlin, Germany. Fritz died in Auschwitz. This information is based on a list of victims from Germany found in the Gedenkbuch - Opfer der Verfolgung der Juden unter der nationalsozialistischen Gewaltherrschaft in Deutschland 1933-1945, Bundesarchiv (German National Archives), Koblenz 1986
Source    
Gedenkbuch - Opfer der Verfolgung der Juden unter der nationalsozialistischen Gewaltherrschaft in Deutschland 1933-1945, Bundesarchiv (German National Archives), Koblenz 1986
Last Name    
LOEWENBERG
First Name    
FRITZ
First Name    
S
Date of Birth    
19/08/1901
Place of Permanent Residence    
BERLIN,BERLIN,BERLIN,GERMANY
Place of Death    
AUSCHWITZ,CAMP
Victim's Status End WWII    
DECLARED DEAD
Type of Material    
list of victims from Germany
http://www.yadvashem.org/wps/portal/!ut/p/_s.7_0_A...earchQuery=&todo=2&images=%5B%5Cleading_folder%5Cgermany.JPG%5D&imagedescs=%5B%5Cleading_folder%5Cgermany.JPG%5D&itemid=3883977&q1=O084lUmdWFo%3D&q2=xPr7GNdYwP%2BmjWX82%2BC7azkv5DoZHypg&q3=FQob5TaG5vA%3D&q4=FQob5TaG5vA%3D&q5=Jm14aeUKcMs%3D&q6=TAoETilmy5Y%3D&q7=uwETJiyoIEY%3D&npage=&zoomdesc=&victim_details_name=Loewenberg+Fritz%3BLoewenberg+Fritz&victim_details_id=3883977&imagenum=0&searchfor=7
Fritz Loewenberg was born in 1901. Prior to WWII he lived in Berlin, Germany. Fritz died in Auschwitz. This information is based on a list of deportation from Berlin found in the Gedenkbuch Berlins der juedischen Opfer des Nazionalsozialismus, Freie Universitaet Berlin, Zentralinstitut fuer sozialwissenschaftliche Forschung, Edition Hentrich, Berlin 1995. 
Full Record Details for Loewenberg Fritz 
Source    
Gedenkbuch Berlins der juedischen Opfer des Nazionalsozialismus, Freie Universitaet Berlin, Zentralinstitut fuer sozialwissenschaftliche Forschung, Edition Hentrich, Berlin 1995
Last Name    
LOEWENBERG
First Name    
FRITZ
First Name    
S
Date of Birth    
19/08/1901
Place of Permanent Residence    
BERLIN,BERLIN,BERLIN,GERMANY
Place of Death    
AUSCHWITZ,CAMP
Type of Material    
list of deportation from Berlin 
* Indicates an automatic Translation From Hebrew
http://www.yadvashem.org/wps/portal/!ut/p/_s.7_0_A...earchQuery=&todo=2&images=%5B%5Cleading_folder%5Cberlin.JPG%5D&imagedescs=%5B%5Cleading_folder%5Cberlin.JPG%5D&itemid=4118152&q1=O084lUmdWFo%3D&q2=xPr7GNdYwP%2BmjWX82%2BC7azkv5DoZHypg&q3=FQob5TaG5vA%3D&q4=FQob5TaG5vA%3D&q5=Jm14aeUKcMs%3D&q6=TAoETilmy5Y%3D&q7=uwETJiyoIEY%3D&npage=&zoomdesc=&victim_details_name=Loewenberg+Fritz%3BLoewenberg+Fritz&victim_details_id=4118152&imagenum=0&searchfor=7
Loewenberg Fritz 
Fritz Loewenberg was born in Charlottenburg, Germany in 1901. Fritz died in 1944 in Central Europe. This information is based on a list of deportation from the Netherlands found in the In Memoriam - Nederlandse oorlogsslachtoffers, Nederlandse Oorlogsgravenstichting (Dutch War Victims Authority), `s-Gravenhage (courtesy of the Association of Yad Vashem Friends in Netherlands, Amsterdam).  
Source    
In Memoriam - Nederlandse oorlogsslachtoffers, Nederlandse Oorlogsgravenstichting (Dutch War Victims Authority), `s-Gravenhage (courtesy of the Association of Yad Vashem Friends in Netherlands, Amsterdam)
Last Name    
LOEWENBERG
First Name    
FRITZ
First Name    
SIEGFRIED
Date of Birth    
19/8/1901
Place of Birth    
CHARLOTTENBURG,BERLIN,BERLIN,GERMANY
Place of Death    
CENTRAL EUROPE
Date of Death    
31-3-1944
Type of Material    
list of deportation from the Netherlands
Language    
DUTCH
http://www.yadvashem.org/wps/portal/!ut/p/_s.7_0_A...%3D&frm1_npage=1