Woz, so sorry to hear of your troubles. I have to say that it's really loads more complicated. You can't change this timestream's past as it's already occurred. Alternate timestreams at this point in time have also passed 2015 -- and typically are extremely similar out many decimal places. Mandela was a huge exception...but rare. Most changes between timelines reveal very minor changes of little import because we are the same and so is our consciousness, wherever we place it.
To truly go back in time on the same timeline is the same as rewriting history and overriding the free will of all beings in existence on this timeline and in this space/time reality. Destroying everyone's past, their memories, all of it. And who's to say what the new reality would be, or what the fallout would be.
If you could get onto another parallel timeline that never came to be or was otherwise extremely different...that would require going to 6D, the place of all wavefunction probabilities in this spacetime reality, observed and unobserved, both the possible and the extremely unlikely, all...then say going to a possible unobserved reality (one that never happened but is possible), maybe your friend would still be alive today and you could pick it up from there if you could inhabit your body there with your focus of consciousness. Again, who's to say what the fallout would be. Unknown really.
Or you could go to the 7D multiverse landscape and go to an entirely different multiverse space/time reality that somehow still had earth and all of us, but where time ran slower since the big bang and it was still 2015. Fallout, also unknown.
I think the only ones that are available to us are the "extremely likely" /extremely similar spacetime parallel branches off our own spacetime tree of 5D probabilities. These are the timelines we can and do jump to, IMO. Probably a vast majority will have the same outcomes regarding your friend, whilst maybe a very small number will not...but I would not hope for a Mandela resurrection...that is very rare. Just rare enough to tip us off to the reality.
OR -- you could meet in spirit -- what many refer to as your "higher selves" and ask her to hear you out and apologise. She should hear you very clearly as she has passed. Many times, if you put the intent out, you can meet in your dreams or in a lucid dream or meditation. Here, you can reveal your heart and offer your heartfelt repentance and ask for reconciliation. Good luck and many blessings -- I think the latter is doable now and doesn't require any advanced consciousness technologies, nor overriding free will and destroying the past and present as it currently exists, for everything else in creation. And you can set your intention to meet with her as often as you like to send love & greetings...and regardless, you can certainly send these to her whenever you like.
Peace & blessings...and much love & light
7L