This is the first post that has made sense of the incident to me. Firstly, I wish the couple well and that all things have been sorted by now. A nightmarish story whatever the wave height may have been.
When I first read this in (I think it was in the Tillergraph), I too was very surprised about the size of the waves. I came to this forum quite late as I was in Australia at the time, and we bought our boat in April. I live in Sydney, Australia, so I have a fair idea of wave height. Also, I used to body surf near Durban, South Africa at the age of eight. One day I was body surfing and the weather turned rather nasty, so I decided to leave the water. I was wading, trying to run (difficult with the drag of the tide on the sand) as the last wave went out, and the water was just up to my shins. I did not look behind me but I heard the next wave breaking just behind. It lifted me right off my feet, tumbled me over about three times, and I held my breath for dear life until it finally dumped me on the sand. Very scary and the episode seemed to have taken a long time. I estimate that that wave was only about three feet. I had seen surf waves from four to five feet around there that day. Fifteen feet is huge and would be classed as a king wave in surfers terms.
Why I mention this is that not so only the height, but the force of a wave that is important. I believe that the fifteen feet description was a typographical error on the part of the reporter, and that the waves were really around two feet but many and very forceful. We also experience bumping just by the wake of a Narrowboat while moored in a calm canal. So a freak storm with two foot wind waves could easily toss the boat about and wreak havoc. It is a very sad story, and it is easy to see how such a size error can be promulgated through copying and repetition.