I think if you just put ground weed on the paper then spread some goo on top of it, you will have hard time making it spread evenly (you will most likely fail). It will also most likely leak to paper too.
If you mix spice with some herbs, first you either want to make changa/enhanced leaf, to make sure that spice is spread evenly on herbs and that you can control dose. Then you try to smoke desired dose of your mix in as few hits is possible, using bong or pipe.
Most likely it's not plant matter that makes your friend cough, but burned (not vaped, but burned by applying too much heat while trying to vape) spice. If you mix your goo with weed, you are adding potential coughing agents, not removing them.
And if you want to get rid of plant material from your spice/goo, it's very easy to just put your goo/spice in shot glass, pour warm naphta over your goo/spice, lets plant material fall to the bottom, then decant and precipitate. You will get white or slightly yellow crystals with minimum traces of plant material.