I don't want to give away who I am in this situation so trying to be neutral.
Katie and John split up 2 years ago. 3 children who live primarily with Katie. John has regular contact, every other weekend and a Wednesday evening. Contact isn't set it stone so sometimes he has them more if work/plans allow. John has a girlfriend, Sarah, who he's been with for a year.
Katie's boiler has broken and John is a Plummer. Katie asked John if he would come fix the boiler as there is no hot water. John initially said yes but then changed his mind and he and sarah both think it's Katie's responsibility to fix the boiler herself as it is her house. John and sarah are happy to support by allowing the kids to bathe at their house until Katie gets the boiler fixed.
Should John help and fix the boiler or should Katie be getting another Plummer in to fix it.