Amending contact schedule for kids for 1 weekend

I have friends who are divorced and have two kids aged 9 and 7.
They share arrangements with kids fifty fifty with weekends being split. One has the children from Friday after school until sat 5pm the other has them from 5pm on sat and takes to school mon am.

My friend is best man at a wedding in July and the kids are invited which means he would like the children on the Saturday during day (he normally collects from 5pm)

The wedding is 150 miles away.
His ex is saying no. He is saying she can have children any other time to make up for lost time but ex just says no.
mediation has been tried in the past and she ends up shouting and mediators have said there is little point continuing as his ex refuses to compromise.

What recourse does he have?
There is no formal contact order in. Place. They have been doing these arrangements for three years.
Can he go to court to ensure he can have the kids for that Saturday?
His children would live to go and as they are both girls are already trying to decide what dresses to wear.

If Mum is being unreasonable and not agreeing to this one-off change in the arrangements and doesn''t ahve a valid reason to not agreeing (ie she has long-term plans for the the children that weekend) then your friend could apply for a Specific Issues Order - but that''s quite a heavy-duty solution for jsut one day/event.

Is it a family member who is getting married, or just a friend of Dad''s? If it''s a family member then it would be unreasonable for Mum to not agree, but if it''s a friend then less so. Children aren''t able to attend every event they are invited to, and if an invitation is extended to children and a parent when the children are due to be with their other parent at that time, then the invited parent needs to be respectful of the children''s time with their other parent.

The person getting married is his oldest friend. They have known each other for thirty years.
Him and the children have spent every new year and easter together with the kids for last nine year.

He is respectful of the loss of time and has offered to make it up at any time that suits his ex.
I appreciate no all events can be attended (ie birthday parties can be complciated) but I would like to think that weddings are more rare.
He has always allowed kids to see their mum on mothers day when he would have liked to have taken them to see his own mum with kids.
His ex has no other plans for the weekend.

Ultimately, this is for your friend and his ex to resolve. Sometimes, despite best efforts to reach a comprise it''s not always possible to have the desired outcome one would like; and your friend may need to face that the children aren''t able to attend the wedding. Presumably if the children don''t go then your friend won''t be able to collect them at 5pm that day.

There may well be factors you are not aware of and that could have a bearing on the situation.

Can your friend not post on here himself?

Thanks for the time to reply.
My friend likes to bury head in sand and avoids confrontation hence the reason I am posting.
I have heard his exes side who is upset that she wasn''t asked first.
She says she feels disloyal to the person who is getting married ex wife''s by rearranging time so that kids can attend.
But she has now said that she will swap days if her ex allows the kids surname to be changed to include her Maiden Name.

So it looks like she is using wedding as leverage to get a surname change.