The issue stabbey, is that the choices you make have repercussions, the drops you get will influence your gameplay, and so on... so even if two people continue on their side, when one of them join his mates game, it isnt really his game anymore depending on the amount of time played etc etc etc
and the prob with my friend was but an example, if he doesnt want to play D:OS for a while, fine by me, but continuing the game alone would kind shred coop from its meaning...
Okay... so for the THIRD TIME, can you answer this: What. is. the problem. with keeping two profiles - one for single-player and one for co-op? Heck, you do not even really need two profiles.
Save Game > "billy ray co-op 01"
Now as long as you don't delete that, you can keep playing with your own character and if billy ray gets the time to play again
you can Load Game > "billy ray co-op 01".
If your problem is that billy ray does not want to (or does not have the time to) play a 50 hour game with you, then you should instead play co-op with someone who
does have the time. No amount of programming in the world can solve the problem of "I want to play co-op with my buddy, but my buddy does not have time/inclination to play with me."
