Frankly, if I were you, I'd do whatever it takes to get rid of the bed bugs ASAP, and worry about splitting up the cost later. The habitability requirement is diminished if the tenant causes the lack of habitability. Unless the whole building is infested, this is probably something you and your infested roommate need to work out.
Normally, the landlord is responsible for taking care of the bed bugs as part of the landlord's duty to maintain habitability. The only exception might be if the tenant caused the bed bug problem.
It is not clear here whether your roommate caused the bed bugs, nor is it clear why your roommate would declare that he would take care of this.
Did the landlord consent to you moving in?
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.
While I cannot give you legal advice in this forum, it sounds as though the issue was caused by your roomate, not the landlord. Thus, the landlord would not be obligated to fix it for you. Get it done and work out a deal with your roomate. You should push your roomate to pay the entire amount.
THIS RESPONSE IS INTENDED TO CONVEY GENERAL INFORMATION ONLY. IT SHOULD NOT BE RELIED UPON OR TAKEN AS LEGAL ADVICE. FURTHER, THIS RESPONSE IS NOT INTENDED TO AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP.