That is something you should have worked out with the artist before they came in with a referral. I guess the question would then be, if you gave the artist a discount, would you get your commission for the sale of the art?
The artist has sold a piece of art to their client outside of your gallery, so you don't get the commission for selling the art (assumption). I would think that a new client brought to you by an artist is then your client for framing.
The way to avoid this is to have a clause in your contract with the artist that offers them the discount only on pieces that are subsequently displayed for sale in your gallery.