Wim,
I have several plants of L. americanus growing in a wet area of the garden, an open drain where rain water runs, so it is generally wet though it can be dry in summer.
In this situation L. americanus seeds itself freely, germinating in the spring following the winter wet when this area would have been permanently wet and even regularly under water. I suggest then that your friend sows the seed into a permanently wet compost, perhaps a pot sitting in a saucer of water or the pot could be placed into the shallow edge of a pond.
I grow L. camtschatcensis in the same area and it doesn't flower as reliably and has not self-seeded to date.
L. americanus is one of those plants which people admire greatly but if planted into a wet area it can become too successful.
Best wishes to your friend's germination efforts.
Paddy
