Perhaps he tried to start running in the wrong way starting off too fast and for too long? We all know that this is an easy mistake to make so maybe it should be pointed out to him that a mixture of running and walking is actually a far better and more pleasurable way to start? This coupled with memories of horrific PE classes at school for those of us who were always the fat, unfit and uncoordinated kid could just be what put this person off...
Try drinking orange juice and/or eating bananas in the days leading up to the marathon to build up levels of potassium. Where are the cramps? If it's in your lower legs and/or feet there is a pressure point where your nose joins your top lip that if you press it relieves the cramp. It sounds crazy and I've never had cramp whilst running or exercising but have had lower leg cramps at other times and it has got rid of it in seconds with no lasting tightness. As I said it sounds absolutely bonkers but if a cramp strikes it's worth trying!
Pippi LS I started doing 1 length at a time and decreasing the break until it was non-existent, soon you'll be able to do 2 lengths no break even if you only manage this once in a session to begin with and just carry on slowly increasing what you can do. I can now do 12 lengths of FC straight off with no break (as long as I don't go too fast!). I've just learnt proper Breast Stroke so I built up just doing Front Crawl. I sound a bit like an asthmatic frog when doing breast stroke but am getting there with the breathing on that one too now! My swimming teacher now says I am doing really good front crawl and breast stroke so I have a schedule to do in the pool, I can't do it all yet but yesterday I did clock up 2km for the first time ever! Once I've recovered from my injury I'm looking forward to learning back stroke, butterfly (why not!) and tumble turns... I currently just do "touch turns" between lengths.
I also like to have a clear run, it can be quite tricky in an evening when there's only one lane and the open swimming area as the lane can be full of fast people who I can't keep up with and the open bit is full of kids, families, chatty ladies... Why do people seem to wait until I'm 3/4 of the way into a length to start swimming at me too? I wear the most garish brightly coloured swimming hat so that people can spot me a mile off and still they try to swim at me!