Giant Marsh Marigold

Asking for a friend as I don’t have a pond, I do have a rather nice wee burn than runs infront if the property so a pond would be a bit of overkill to be honest.
A neighbour has a stunning clump of Giant Marsh Marigold that bloomed it’s head off for weeks, just starting to stop now. It is in an old tub with no holes in it…yes not glamorous. Neighbour is NOT a gardener!
A friend has just created a new large pond and horrified at the cost of pond/ marginal plants. I have asked neighbour if I can have a chunk of the Marigold, answer yes. Does anybody know when I should do the division?, presumably when it is dying down or maybe early spring? How aquatic is it? Should it be planted in the pond like a water Lily or is it more of a bog plant. I know zero about this plant other than it really makes a fantastic statement when flowering.