one of the highest causes of accidental death is drowning from shock/immersion on cold water. I think the most logical explanation at this point is that NB sits at the bench after letting the dog off the lead, leaves her phone on the bench and for whatever reason is near the river bank, who knows why, maybe a ball? Maybe just to stand near the edge? She either slips/falls and lands in the river. It would have been freezing at that time of day. The dog may not have seen hence doesn’t jump in. Undercurrents mixed with shock/panic/potential medical episode would result in drowning. The idea someone else is involved is slim, there were plenty of walkers/witnesses around, broad daylight and it’s not exactly easy access in and out.