I am trying to figure out what you are asking.
Do you mean that someone has a gf, and unknown to him a user online is actually that girlfriend? And she, pretending to be a third party, urges him to spend a lot on his gf, and then offers him the chance to be unfaithful to his gf (since he would not know who she was), as a "test?"
If that's what you mean, my answer would be "why would a stranger online have any say in how I treated my partner?" And if someone found out that his gf was doing that, he might want to reexamine his relationship with her.
