Ok here's the response from Nicole Dupuis, Director of the L’Oréal Canada technical center:

She can use the Brass Banisher from Colourist Secrets, as it will help get rid of the yellowish, brassiness. This product is gentle enough as it does not contain ammonia. Rinse the product after 5 minutes and then wait 4 to 6 weeks to colour again.

I hope that helps!