One guess:
It might be a misunderstanding. Maybe she found accidentally something unpleasant or really disgusting from browser or google history, such as child pornography, zoofilia, necrophilia, ... Katya was quite a long time really shocked before she started to cry.
However, she came back over to Ruslan, so that what a computer was, was probably a misunderstanding. The boy who visited their place might be the culprit. He used that computer at every night.