The drinker is responsible.
See SA CM 182.1 SMA sk2
חוץ מלדבר עבירה: דדברי הרב ודברי התלמיד דברי מי שומעין ויכול המשלח לומר סברתי שלא ישמע לי לעשותו לכך אין המשלח חייב אבל אם אין השליח בר חיובא לא שייך האי טעמא:
There is no shlichut for a sin out of for an Avera. Because the Meshaleach can say, I was thinking that the Shaliach will not agree to make the sin.
So if someone says "Take it, it's mine." There is Shlichut according to the SMA.
But anyways, in your case the co-worker didn't say, "Take it." There is no Shlichut. So, the Gezel is only the responsibility of the taker.
I understand from comments (thanks to @DonielF and @DanF) that my answer isn't clear enough.
In the first part of the answer I wanted to exclude the lack of responsibility of an eventual "Meshaleach" according to the rule written in SMA, despite the known rule of "Ein Shaliach lidvar Avera" (there is no shlichut for a sin)
In the case of the of the OP, even with the rule of the SMA, the co-worker is not responsible as explained above. The drinker acts following his own willing. (To understand this I recommend learning the first mishna of the 6th chapter of Me'ila). This Mishna talk about Meila, an exception to the rule of Ein Shaliach Lidvar Avera. The cases described in the Mishna define when there there are truly a function of sender and a function of agent, and when it is not the case.
If an agent has fulfilled his agency, the sender is guilty of sacrilege, but if he has not carried out his agency, he himself is guilty of sacrilege.
How so? If he [the employer] said to him: “Give meat to the guests” and he offered them liver, “[Give] liver” and he offered them meat, he himself is guilty of sacrilege.
If the employer said to him: “Give them one piece each,” and he said to them: “Take two pieces each,” and the guests took three pieces each, all of them are guilty of sacrilege.
If he [the employer] said to him, “Bring me [something] from the window or from the chest,” and he brought it to him [from one of these places] even though the employer says, “I meant only from that place” and he brought it from the other place, the employer is guilty of sacrilege.
But if he said to him, “Bring it to me from the window,” and he brought it from the chest, or “from the chest” and he brought it to him from the window, the agent is guilty of sacrilege.
- So the drinker made the sin. The rule regarding stealing, is that one needs to pay, wether he is aware of the sin or not.
As @DonielF noticed, in Bava Kama, stealing is included in some Berayitot as av Nezikin and a man needs to pay dammage even if he didn't it intentionally. (There is an exception in Yerushalmi if someone place an object near a sleeping man and this man broke it in his sleep movements). Even if when saving oneself from a murder someone broke utensils of his friend, he needs to pay meykar Hadin (see BK 117b and Rashi ולא מן הדין. שהרי המציל עצמו בממון חבירו חייב כ''ש מציל אחרים בממון חבירו: )