I agree with you on this, TurtleDove. The focus should be on the outcome, which is a fact, rather than the behaviour, which can be speculation. So: Flirty is not getting her work done, which leads to OP working overtime, rather than Flirty is chatting/spending too much time with other people.
(Also as an aside, the latter version seems to blame Flirty for something two people are doing -- Flirty and the guy she's talking to. So it might seem odd to a manager that the OP is only focused on one person. Remember, the guy she's talking to has agency here. He can say "Flirty, I have to get back to work now." He's choosing to chat, too.)