Just to clarify, are you talking about instrument supported upgrades or CPU? I will assume the latter since Plat don't get many PP. And you want to pay to upgrade your wife because she needs the PQP and wants to keep Silver? That means she will have to be on a separate record, so splitting her out is a must. That leaves you two options...keep you and your other friend together, or split.

Remember that when you get to 24 hours, you will need to split your record to remain on the upgrade list anyway and then tell the check-in agent that your friend is your companion to hopefully get them your status in the upgrade list. So, my suggestion would probably be to keep you both together in the hopes of an upgrade at the PLT window which is admittedly rare these days but maybe if there are tons of seats. And then split when you get to 24 hours and try to add them as your companion (and do the same on the return at check-in)

I'm not sure why the upgrade price for the friend on a separate record is relevant to the rest. It likely costs your wife more to upgrade based on the current fare class she is booked into, so leave that to the side.