Straighten an old perm

I am a hairdresser and a client of mine wants her hair straight.  About 2 years ago we permed her hair.  And now she would like out.  She does not want to cut it all off, very attached to her 30ish inches of hair. 

 

I know you can re-straighten it with a perm but I am not sure how to do it.

 

Her hair is naturally blond, but she has done at home hi-lites.  She does regular deep conditioning treatments with professional products..

Help please!