I have a similar situation with a husband who flies much less than I do.

When we go somewhere together, I buy his ticket with my miles, then buy my own ticket with cash (credit card). That way we get a free award ticket and the flown paid-for ticket puts miles into my account.

I've done the same when young relatives have needed help with a plane ticket. If I am otherwise inclined to buy them their ticket (indulgent Aunt), I get it with my miles.

Bottom line: The miles accumulated on a paid-for ticket go to the account of the person sitting in the seat.