Also you could take some of their gravel if they are using gravel.then hang it in a net in ur tank this has good bacteria on it too.
Unfortunately, this will do next to nothing compared to direct introduction of mature media into the new filter, so is not really necessary assuming you are able to follow through with the media. In cases where someone can't get the media this sort of action is better than nothing though.
Totally agree with advice in this thread that the key things are to not remove more than 1/3 of the biomedia from the "donor" filter and to already be dosing the new tank with 4ppm or so of ammonia concentration for the bacteria when they arrive in the new filter. The usual etiquette when obtaining mature media from another person is to purchase a full set of new media for them, from which you cut/divide enough to replace what you are taking. Keep the donated media in tank water, typically in a plastic bag, when moving it.
~~waterdrop~~