Even though she has been sexually assaulted and threatened with a firearm, SHE is the one who reported him - therefore a "grass" - granted not a nice description but :-
"Definition. a person who informs, usually on criminals. (British, slang)"
(In this particular case, it would appear that "victim" could be substituted for "person", but let's test a conspiracy theory:-
Girlfriend has been "bought" from a bar, and falls out with the boyfriend over money, so says she has been sexually assaulted. Now, correct me if I'm wrong, but are sexual favors not what bar girls provide? The fact that he has firearms all over the place gives her an "edge" so she "grasses" him up?
Personally, I'm glad they caught him, the only Russians (or any other foreigner) that have firearms shouldn't be here - their country is at war, and if he has such a liking for firearms I am sure that nice Mr Putin could find him a job?