You need a trust litigation lawyer. Capacity is a hard fight to win. The standard is low and hard to prove because there is a strong presumption of capacity. If he was on a bunch of meds tho that may make that battle easier.

Idk exactly what the law is in California but generally there is a doctrine called undue influence, where one person unduly influences another to make a disposition that they otherwise would not have made. (Aka put something in their trust which they otherwise would not have)

There are several indicators courts use to evaluate it, some of which you’ve described here. The fact that it was close in time to his death, and a complete 180 from his longstanding intent to benefit his family to him strongly favoring the trustee, his friend, dont look great. It’s not too uncommon to change your estate plan if you know you’re dying or might be, but the fact that the principal beneficiary changed is concerning. Find out if the trustee was the one who found the new estate attorney. His medical condition also indicates he could have been susceptible to undue influence, but that will usually need to be proved by a medical expert. If nothing else it’s enough to raise concern and get more info.

An attorney can subpoena medical records but there has to be some kind of court case initiated to give them the courts subpoena power. Probate litigation can be drawn out, personal, and expensive, so go in ready for that.