As F/O you should know perfectly the technical part of your bird but included limitations, MEL and procedures. This is of very valuable help for conflict like you had to face. Let assume that you are on gnd and the WX radar is inop, with the MEL you have a tool to hand to the CDR and take apropriate decision. Then in your OM it must be explained how to deal with CB and WX radar, you should be able to reach the information quickly and read it to the guy who is wrong, then he might not say you are wrong. I know it is a lot to know but you have to, especially if you are new in the job. Remember that you have the power to say it if you are not happy with what you see, complacency can kill you and if you go by the book, you will not be blamed.
If you feel uncomfortable with this CDR, it must be for something, talk with your chief pilot before it is too late!