As a general rule, you don't incur personal liability as a trustee simply for occupying the office of trustee, although the trust estate of your friend's revocable trust would be responsible for his debts and liabilities. That doesn't mean that you would be insulated from involvement, however, if a lawsuit were filed against your friend and his property were sought to be made responsive as a recovery source. You would be joined in that action at some point, or subsequently in a judgement execution proceeding, and while the trust would be available to defend you as trustee, this is still a concern and stress factor for you to consider. Additionally, if there is some possible subterfuge taking place, a plaintiff in pursuit of your friend could try to join you personally as being somehow involved in a civil conspiracy to injure your friend's creditors.
Being a trustee is not a role to take on lightly, and doing so gratuitously for a friend is magnanimous; you should know what you are getting into. If your friend should die--and eventually that will happen--with you as trustee, there is work to be done involving post-mortem trust administration, or at the least, you would have to resign, and if you delay, then did you marshal assets and protect the trust in the meanwhile? Since you are already serving, you couldn't say that you never accepted the trust and commenced to serve upon your friend's demise.
I suggest you call one of the local Claremont-Pomona area probate lawyers and review your situation and the facts as known to you with that person. You may conclude that you would prefer to "pass" on this opportunity to serve gratis as your friend's inter vivos trustee.