Inheritence Planning – US Assets

Seeking a bit of advice on how to approch this – thank you in advance.

My partners (a Japanese native) father is liekly to pass on due to ongoing heath conditions in the very hear future.

While he has domestic assets/income such as a Japanese retirement fund, money in the bank etc – his major assets are oversees in the States (of which he is a national). One is a 401k and the other is a American Fund (Capital Group).

What should I be prepping? How to I ensure that the maximum value of these assets/income is recieved and how would I go about enabling this?

Thank you in advance!

by NomujoaJPN