If your friend has a lawyer, that lawyer could make arrangements for a notary to visit your friend, get a signature, and notarize it.
The only other thing that you can do is contact a notary public in CA and hire him/her to go to the jail and do the same thing. You, or he, could probably retain a lawyer to just do that and it shouldn't cost much.
I believe it when he says that the jail won't provide notary services. I have a hard time believing that the jail would not "allow" notary services.
I hope this answers your question. If there is anything that you do not understand, please let me know and I will be happy to explain further.
In the meantime, please click "Accept" so that I may be paid for my answer and any subsequent follow-ups with you.
P.S. I am frequently asked how someone can specifically request that I, personally, answer a question. Here’s how: for new questions in the future, always begin your question with “QUESTION FOR THE JUDGE.”