There is a story in the AA big book which always resounded with me, about a man who waited until his retirement to drink & then experienced a rapid descent into alcoholism.
I was clean & sober for many, many years while raising my children. When they grew up & my "nest emptied," I returned to wine to fill lonely evenings. This stage of life is distinct & challenging. If your life had a very solid form, the formlessness is disorienting.
This does happen to some very good, kind people.
I send many sympathies to you for your loss of a father. Your heart must be breaking & you must be very worried.
I have no advice. I found myself & a renewed identity in recovery & work. This might be a moment of disorientation or it might be his story. I agree with many who believe that meth is a powerful & soul-sucking drug which changes people.
You may have lost him & that is heartbreaking. You say you are posted far away - perhaps al-anon is not available where you are. You may have to work hard to find a support community.
You are just witness. There is nothing you can really do but wait & send hope energy (perhaps in the form of prayer). But for all your worry about him, please take time to care for yourself, to grieve, and to release the end-story (as you cannot change it). You can be present for this & can feel it fully. That's all.
A vast hug to you! This makes no sense & is really hard...
We lose good people to addiction sometimes, and it is a very deep & confusing loss.