The moment she decided to fuck someone while he's not there, she was no longer loyal to him in my book. I didn't say she cheated, but he's obviously no longer enough for her and she needs someone else to feel her... gaps. I'm not into polyamory, so the fact that she's "keeping it in the family" doesn't change much for me. It's just personal preference. Like I said, I'd just wish to see a love interest who keeps proclaiming her undying love for the MC, actually proving it with her actions for a change.
I do have to remind you, that it was the player's decision to buy the toy, so what you're saying is at best arguable.
After all, it is a optional event and not forced on the player. So if she is really not loyal, it is so because of your decision.
The toy's description are "Something for those more lonely moments." "Unlocks new Event. (Violet and Nina)"
At the same time, Nina confessed in front of the player the first time Violet and Nina met, that she will share the same love to him and Violet.
So discussion about NTR and not being loyal, for my point of view is just nonsense.
Other characters events that lead to a female eating out another female has been always optional and player decision based.
Violet is a unique waifu and is always loyal to the play no matter what.