I agree with the will/letter idea. A lawyer can keep both and then present them to Diane at the appropriate time. There's also keeping a letter in a safety deposit box, but that means a key has to be secreted and the place where the key is kept has to be known to Diane.
it has happened that tearful widows
show up, requesting access to safe deposit boxes.
Bank personnel inquire qua the nature of the trouble and she blurts out that fellow-boxholder is dead,
whereupon she is REFUSED access
to the box (maybe for several weeks)
until the IRS
and/or miscellaneous state officers have a chance to LOOK IN the box
, and audit its contents,
to their hearts' content, on behalf of GOVERNMENT.
In the meantime, SHE IS SCREWN.
She can 't even lay her hands on her OWN
personal stuff in the box.
That has happened too many times in American history.