True refundable tickets are refundable post-flight in most cases. You will hardly be the first to have done this and you should have no issues or concerns if the circumstances are how you described. .

There is a known case of a GS having done this regularly, multiple times for a companion and that did lead to suspension of their account

If using the 24 hour refundable policy, there is one reported case of UA refusing the refund (suspect their were other circumstance involved).