I know the weather is too hot for snowflakes in Australia but the climate in the UK means we get a few.
Firstly, I said 'potentially'. There's no obligation for another parent to 'assume responsibility' as stated by another forum user nor, is there an obligation on the coach either. So, if you send away the parent (and good luck with that on a public field) there's a possibility the child may have to go as well. A little unfair on the child won't you say? (You can choose your friends but, you can't choose your family).
Secondly, we seem to be a confrontational bunch on this forum but in these situations I'd rather get the home team to do some legwork before making it all about me.
And yes by all means abandon if all else fails and certainly report if the miscreant is sent away and goes. My view is that you should report as a welfare issue irrespective of the parent leaving our not. Sometimes the pen is mightier than the sword.
There isn't an obligation, and neither did I say so. Some kids don't even turn up with their parents, is there then a welfare issue before they've even started?