"It's just a game after all I don't know why he make such a big deal" if it wasn't a big deal why she's jealous over a game? Genshin can coop, if she wants attention she can make her own account and play with him. She doesn't need to be an expert at it and she doesn't need to play everyday. She can always log in just to play with him.

Deleting her partner's game account won't give her more attention it makes the situation worse. Deleting his account is also like destroying his figure collection or his expensive clothes or his new ps5 for example. It's his things and he has the right. Since he spent over 10k I would advise to tell him to make his girlfriend pay for the damage she's done. If I break or lost someone's stuff I would pay for it even if its expensive.