I'm so sorry that you lost your Lovebird, what a horrible thing to find and have to deal with. Accidents happen like this with parrots and water all the time, usually it's an open toilet seat/bathroom door, as once they get wet they can't get back out, and they tire quickly and drown. It's a terrible thing to have happen, so my deepest condolences to you and your family...
***As far as getting a "new mate" for your other lovebird, the answer is a resounding NOOOOOO! You should never, ever, ever buy another bird for your current bird, this rule applies to any and all situations! The only reason that you should EVER bring another bird into your home is because YOU WANT ANOTHER BIRD!!!
AND WHY IS THIS? ****Because there is absolutely, positively, 100% NO WAY FOR YOU TO KNOW WHETHER OR NOT YOUR CURRENT LOVEBIRD WILL BOND WITH WHATEVER BIRD YOU BRING HOME, OR ANY OTHER BIRD EVER AGAIN!!! Just because she was bonded with Oliver means nothing in-terms of her bonding with another bird ever again, and certainly not with whatever bird/Lovebird it is that you bring home for her.
Parrots are very much like people when it comes to personal relationships within their own species, they might hate another bird they meet, they might simply tolerate them but not want to spend any time with them, they might like them a bit but not bond with them closely, or they might love them bond closely with them. But there is no way for you to predict what will happen if and when you bring home a new Lovebird.
I can tell you not from my own, personal experience, but rather from people in just the situation that you are in who have posted on this forum asking the exact same question, that this rarely ever works-out, and then the owner inevitably posts "So now what do I do? I didn't want another bird, I got this new bird because I didn't want my other bird to be lonely during the day while I'm at work", or "I got this bird for my other bird because his bonded-mate died, and they hate each other, and I don't want another bird for myself!"
So, the bottom-line to this situation is ONLY bring home another bird if YOU WANT ANOTHER PET BIRD YOURSELF, as it's very likely that your current bird will not want anything to do with the new bird, or vice versa...
****VERY IMPORTANT****
If you do decide to bring home another Lovebird, DO NOT DO-SO WITHOUT HAVING A TOTALLY SEPARATE CAGE SET-UP FOR THE NEW BIRD, COMPLETE WITH IT'S OWN DISHES, TOYS, ETC., AND HAVE A SEPARATE PLACE TO HAVE IT SIT IF NEED-BE!!! You certainly can never, ever just bring home a new bird and plop it in the same cage as your current bird, that can and probably would result in injuries, death, plucking, behavior issues due to stress and anxiety, etc...PLUS, you must always quarantine a new bird away in a separate room from your current birds for 30 days anyway. But the real issue is that you cannot just assume that they will bond, because it's likely they won't, and if you don't have a separate cage all set-up and ready for the new bird on the day you bring it home you'll be in serious trouble...But again, unless you truly want another bird for yourself, then there is no reason at all to bring another one home for your current bird. Just be sure to give your current bird plenty of love and extra attention and help her through this tough time. She'll be fine...