Usually when a holy war is declared, nobles with the same fate as the defender will join the war alongside him.
Will catholics help an excommunicated catholic, if I, as a cathar, will declare a holy war against him?
And what happens if along the way his excommunication is lifted?
Yes. An excommunicated Catholic is still a Catholic and nearby Catholics can still join his war- it doesn't matter if they're under attack by a heretic or a non-Christian.
They are, however, a lot less likely to. Joining a war is based on opinion- since excommunicated characters get -30 opinion with all Catholics, his neighbours probably hate him at least as much as they hate you for being a heretic.
You might also be able to attack while some of his neighbours are already attacking him for being excommunicated- in that case they can't join him.
