To encourage my son to break a friendship and contact friends parents as to reasons why.

I would definitely call that mum even if nothing comes out of it. It would at least demonstrate I care. Your son sounds isolated and desperate for support

If it becomes bullying, contact the school. They have bullying policies in place and an established system to handle it.

The OP should not ring the parents. There won’t be a productive outcome and it could well make things worse for her son.

At the moment I understand it to be one boy slagging off the OP’s15 year old to his girlfriend, claiming he’s a liar and will hit her. That’s an issue for the lads to resolve between themselves.

No, not school’s problem. They have enough to do.

Do not get the mum involved, there's not much she can do even if she believes you (which is unlikely). Get DS to think through how it will go, that'll help him realise how daft an idea it is - nobody has ever decided they like someone more, and will be nicer to them because they told tales to his mum and she told him off.

Instead encourage your DS to think about whether he actually likes this boy, a friendship is not some sort of contract you can never leave - it's a consequence of mutual liking and respect.

You really think that the other parent can fix that even if they believed OP’s son?

Police and school are the only ones who can really help.

If he is old enough to have a girlfriend I don’t think you should be getting involved yourself . Talk it over and offer advice.

I would not call the parents. This isn’t how fall outs between 15 year olds are sorted.

Explain to your son that contacting the parents won’t help and that it’s not like being 5 and an apology meaning that you can be friends again. Whatever this “friend’s reasons, I think that you need to explain to your son that this is not how friends behave. I know it’s hard only having one friend but it’s best not to have people like this in your life and he’s at high risk of this happening again and again.

He’s a child? You wouldn’t protect your child against being attacked by a gang of boys, and your quite happy to have one going round spreading lies that he beats up his girlfriend??

Op this boy is not your sons friend! He sounds like he’s really got it out for him? With the malicious lies and the potential of the planned attack to be true, I’d be considering contacting the police. Your son needs protecting. I would be contacting the parent, there’s nothing you’ll be able to say to ‘fix’ their relationship, as their relationship is one of bully and victim. I’d be laying it out to the parent like you know the lot to be facts. That her ds has been spreading malicious lies that your ds assaults women, and that her son was planning on trying to get all his friends to jump him. If this doesn’t stop immediately you’ll be contacting the police

I would report the bullying to school.
Investigate whether there are any underlying issues causing the lack of friends eg neurodiversity, self esteem issues. Encourage DS to block and ignore and try and find new friends. Encourage DS to join activities outside of school with different people eg Scouts. Considee changing schools. If I could afford it I would get DS private counselling to work through this and strengthen his social skills. If not I would be asking school if there's anything they can do to support.

Absolutely not. Do not ring the parents. At 15 your son needs to manage his own friendships.

I’m surprised he even asked you.

Don’t make any calls and get your son to have nothing to do with him this isn’t a relationship to fix.