Come to think of it, what kind of bottles is this guy using that a toilet brush even fits into them? When I think of a toilet brush, I think of something with a very wide handle and head: not the sort of thing that would fit into most bottles. What I think of a bottle brush has a very thin, flexible handle, often made of twisted wire or something similar.

One possible solution to this issue would be to get the guy a bottle brush, with the stated reason that it will make cleaning his bottles much easier. This avoids the awkwardness of admitting that his old brush has in fact been used for the toilet (which, as I stated above, I suspect he already knows), but it's still true.