Hi Kay,
Please DO NOT open a joint account with your client. Here are some options to pay your client’s bills:
Pay bills online via logging in directly to the vendor’s website (like the Gas Co, Edison, Spectrum, etc) and pay from there.
Open a separate checking account in your company’s name that will be funded by your client and will be used solely for his/her payables.
Some banks allow you “bookkeeper” access, which gives you limited authorizations without having you co-owning the account.