Are you going to share ad infinitum?
Until his investment is paid out plus X? What?
Depending on this, you could simply do a variation, i.e. if you offer him a payout with interest, simply pay him everything after costs until he is paid off, then seperate the deal and only mine for yourself.
If you offered to pay him a share forever then if you are really being fair, you might give him the equal proportion of income from the rig he paid for 50% after costs, i.e. if he paid for 50% of rig II, he gets 50% of profit from rig II, too.
Whatever you do, make sure he is over 9000% content with the solution however.
As has been pointed out: money and friendship is rarely a good idea.