I wonder if Himmel didn't want to add more to her burden as a immortal being by getting into a relationship with her, just to leave her alone afterwards...
A. Himmel wouldn't have known that, he can't see the future
B. From the flashbacks that we see, Frierin hasn't changed significantly in the past hundreds of years since we saw her village being destroyed and learning magic from her teacher (not to mention that it also means that elves or atleast Frierin, have very good memories) so remarrying might not be a thing for her