Not sure exactly what the "finder fee" means, but where I come from, you would often submit your invoice to the graphic designer, and the designer would pay you and then add a % onto the client's invoice. (So if you charge $60 / hour to the graphic designer, she will charge the client $80 per hour for that work, for example.) So from your point of view, it should be irrelevant what the designer has organized with the client. Presenting it to you as a fee to you seems like poor politics to me.
Personally, I dislike this kind of arrangement anyway. As sagewing says, you are allowing the designer to get work she might otherwise not have gotten by filling in the gaps in her abilities. If might be fair for you to drop your price a little in gratitude for getting the work ... but that's for you to work out.