Dealing with suspects of sabotaging experiments
That would be the time for the PI to call a meeting with A and B and have a conversation among adults. Topics to be discussed are: If A has a grief with B, she needs to be specific and put it on the table. If B suspects A, he needs to do the same. PI needs to say that this is unacceptable and that everyone needs to behave like adults or else.

If the PI does not call this meeting, then either A or B need to be the grown-up in the room and do so.

Ultimately, half-truths, allegations, conspiracy theories, "experiments not working well", etc, is a situation that is to nobody's benefit. The only way to address it is to talk about it.