Taking someone else's cancelled German visa appointment slot - realistic?

Does anyone know how soon the time she cancelled appears as available if I want to re-schedule mine?

I admit, I have no idea. However I suspect no one knows the answer. We do however know that, at least for some countries (not necessarily Germany) appointments are released in batches (but I can't find a link for this at present). So the time interval would then depend on when your friend cancelled relative to when batches are released. You mention I am aware that it takes 1-2 days to delete my info from embassy's database. This demonstrates the system is certainly not all "real time".

But even if not in batches availability may be tailored to demand. That is, your friend's freed-up slot may never become available to you, or anyone, if it is at a time when interviewers are expected to be in shorter supply than previously anticipated. Not for Schengen but the principle is as described by immihelp:

appointment availability changes all the time as new slots open up, people cancel and reschedule their original appointment and other circumstances such as demand changes

If the centre you chose for interview expects to have say 10 interviewers available they may offer say 60 appointments per day, your friend's being one, booked some weeks ago. Since the booking maybe one or two or three of the interviewers went sick, or was dismissed or failed to make the expected return from maternity leave. Capacity at the planned rate has reduced from 60 to 54, or 48, or even 42 whereas booking may only have dropped by 1. There should be some slack in the system but it may not be enough to accommodate such imbalance and the simplest way to adjust is to start by holding on to any cancellations – without releasing them to others, you included.

Swapping slots should be discouraged anyway, hence there is incentive for the system to make doing so difficult for you. Otherwise I would expect "slot squatters" exploiting the opportunity to exchange their 'early' slot to someone in a hurry (and the money to lubricate such exchange). In due course for them the slot that was yours would be 'early' for someone else in a hurry, and more money change hands. This would also be a reason for embassy officials to be disinclined to approve an 'official' straight swap.

Regarding your Comment I hope there won't be anyone fishing for time slots at 2am you can hope but there were 2,022,870 Uniform visas (Germany) applied for in 2015. That is well over 5,000 a day (across all centres) yet it would only take one to stymie your plan. However desperate you are, it is quite likely that there are others more so, or night shift workers, or insomniacs.

realistic?

Definitely not, more harebrained.