Take a look at the technical discussion in a recent thread.  Your client is kinda in a bind.

CT changed it's law about two years ago to tax only NR telecommuters from the states that apply the so-called convenience of the employer rule as a **bleep**-for-tat strategy.  They did not, however, change their regulations on other state tax credit to source their own residents' telecommuting compensation to their employers' state so as to allow credits to be claimed.  This means residents of CT telecommuting with a NY employer but not for the convenience of the employer can only claim credits on the CT return for actual workdays they have in NY but not the telecommuting days in CT.

https://proconnect.intuit.com/community/proconnect-tax-online-discussions/discussion/re-client-works...