So sorry to hear that, Paul. It happens to us all and it's always distressing; you always wonder, "Did I miss something?"
It was a caddy lamb - had it had any vaccinations? (Heptavac, Covexin, or similar.) If it had then it's probably just one of those things, would have happened to anyone. We get an occasional (one or two a year from our 270-300 ewes) strapping lambs keel over and die suddenly; BH usually says it will have been heart attack.
I hope your daughter will not be put off keeping livestock; it's horrible to lose an animal you've cared for however old you are. Best to 'get back on the horse', if possible.
Best wishes to you and your daughter
Sally x