I understand how you feel because I love crediting what other people did. Not knowing who's the simmer behind a sim can be lame.
However, I agree with most comments. The beauty in The Sims franchise is that we are willing to share our creations for others to play. If I were to creator of your sim's girlfriend, I'd be thrilled to see how she's doing in your game.
I myself intend to share a couple once they finish their life in my game. Like, give them young again potions and pack them to be shared with a few tokens. I'd love to see the different children and lives they could have.
So, don't feel guilty. I bet the person who created that sim will enjoy seeing you play with her.