I think its totally irrelevant who pays how much towards the flight where you are not carrying anyone else. Or carrying someone who is not contributing to the costs of that flight.
If a friend/colleague or family member or kind benefactor gives me some money and I choose to spend it flying then that is up to me.
It is also totally irrelevant where I choose to fly.
If I choose to spend the money on a meal/hotel present for the wife etc. instead, then that is also up to me.
When your friend gets on board and you split the costs that flight is subject to the rules on cost sharing. The flight before and after when you are on your own or with someone else who is not contributing towards the costs is just flying like every other time you fly.
All in my opinion of course.