Extra Recommendation

[ends_and_means]

I'm currently applying for a Master's in Writing with a concentration in Publishing; I have my three required recommendations (all former English professors). However, my mother's friend is a professor at the university I'm applying to -- granted, she's in the French department, not English. She's read a lot of my writing and has offered to write a recommendation letter for me. I don't want to be rude and refuse, but... isn't this generally looked down upon? Or will it not matter much as I'll already have three legit recommendations?

Opinions, please!