Which variety has he planted? Generally, they do spread far and wide, and once they do, its impossible to stop them, so unless he's got, say, Fargesia, which doesn't spread quite so much, he'd be better off heaving it out of the ground and either growing in a very large pot, or putting a rhizome barrier into the ground to entirely circle the area he wants the bamboo to fill - its a performance to do, because it means excavating the soil down to 18 inches, inserting the barrier vertically all round, replacing the soil and planting in the middle. But time spent doing that now will save 20 years of trying desperately to eradicate bamboo from the lawn and other side of the garden... The other option is one of those small dustbins, about 2 feet high - cut the bottom out, insert into the soil, plant in the middle of that, but that only works if its wide enough to fill the gap he wants to fill.