I'm a qualified Youth Worker with a degree in Youth Work, working in the field, with responsibility for Safeguarding issues. The fact that the contact with your daughter has come from an official organisation is the factor that means this is a safeguarding issue. If they had met at school and were going out he would be being incredibly stupid and ought to know better but it's a different category of problem.
If this man is on a course where he is learning how to work with children in any capacity, he should have been taught about issues like Abuse of Trust within the first week or so. If he is being employed by any kind of organisation working with children, he should have been given training by that organisation, given policies, and CRB checked. These two things mean he should know that what he is doing is not allowed. If either the course provider or the sports organisation has failed to do either of these things then they share responsibility.
You must report him, and yes it will ruin his career, but that is why those of us who work with children and young people work so carefully to be transparent and above reproach, so we are demonstrably accountable for our actions. If an 18 year old male youth worker starts to receive texts from a 14 year year old girl he should have immediately told his supervisor, shown the texts, and you your daughter should have been spoken to and asked to ensure the texts stop. In reality he shouldn't even have given her his mobile number, nor asked for hers.
The other thing I would say is that you should report him to his organisation through official channels (Ask to speak to their Safeguarding Officer - they should have one) so that there cannot be a cover-up. It is a complicating factor that his father works for the same organisation, but the organisation shouldn't be allowed to be swayed by that. You should keep all the texts, and not make any further attempt to contact either the man or his family again.
To be honest I'm wondering whether it's a police matter - I've just sat here for the last 40 minutes pondering what I would do at work - I think if a young man on my team was reported to me for this I would want to take some very high-level advice about whether the police should be involved, and I suspect the advice would be that they should. If you wanted some other advice you could ring Childline (parents can ring too) or your local social services Duty Officer, which you can do anonymously.
When your daughter has calmed down you need to explain that she hasn't done anything wrong - that this man who she trusted was misleading her and shouldn't have been behaving this. You could talk about how adults who work with young people should be trustworthy and protect them and what he has done has betrayed this trust, and that he might be doing it with other girls too. I know she might have been a bit unwise in what she did, but don't bring that up until the dust has settled - the main thing is that she knows that she wasn't to blame, despite what he said, and that she hasn't been harmed beyond having her feelings hurt and her trust dented.
Sorry - immensely long post, I hope it helps you as you and your family go through this.