This is unfortunate as it sounds like you all have a pretty good lottery pool going on. Scanning the tickets and sending them to everyone before the drawing is good practice. I would also be sure that everyone has read and signed a written agreement before each drawing which is signed and dated by each participant and clearly states which draw date(s) the agreement represents. We are talking BIG money here and if your pool was to win, there could be significant backstabbing without good documentation of the agreement (I personally do not participate in pools because of this reason). Even with a well done written agreement a good lawyer could be hired to dispute who gets what.
In regards to the "ticket buyer" from your pool buying $50.00 worth of EXTRA tickets for each pool participant.... This is extremely uncalled for in my opinion and I would be inclined to stick her with the bill and tell her to keep her tickets. She did not have the authority to spend yours or anyone elses money. Chances are, none of those tickets will be winners anyway and she will hopefully learn her lesson when she gets stuck with 100's of dollars worth of tickets. On the other hand, on the off chance that one of those tickets is the winner, you would NEVER forgive yourself for not ponying up the 50 dollars. On the bright side, paying the 50 dollars will get you into a huge pool of tickets for the biggest single Jackpot in the history of the world! After taxes we are talking approximately 800 million dollars split between 27 people means you would be entitled to about 29.6 Million dollars.
If all 27 people in your pool are contributing $50 that means each person will contribute 25 "plays" to your pool. That is 675 tickets or "plays" in total. This means your chances of winning the Jackpot go from over 292 million down to 1 in 432,891. To put that into a more clear context, your new chances of winning the Jackpot are 0.000231% (Still Extremely Unlikely).
Ultimately it is up to you, I would consider your current relationship with this person and how it could impact your work life if you decide not to pay the $50. The path of least resistance is to pay up and move on. I can tell you that I would gauge what others in the office are going to do and probably go with whatever the majority does. I would also demand that a written agreement be made for future draws and that if the ticket buyer decides to spend money on other participants behalf without asking those tickets are automatically included in the pool and the ticket buyer is stuck with the bill; I would say that would keep people from doing this in the future.
I would also be kinda upset that this wasn't a last minute purchase, why she didn't ask the group today and then buy the tickets tonight, tomorrow, or Wednesday is beyond me. Again, it's ultimately your decision and just thought I would provide you with an outsiders thoughts without making the decision for you.