You should feel guilty for cheating on your girlfriend with this random girl who you met on internet as you should have stayed loyal to your girlfriend. However, if you wanted to have sex with this random girl then you should have broke your relationship with your girlfriend before having sex with this girl.
However, here you feel guilty because you came to know that this girl’s father is no more with her so you feel petty on her and you feel that you did something wrong but one thing you should understand that she wants to have sex with her and it gives her pleasure and she has a right so you have not done anything wrong by having sex with her and you should not feel guilty for that.
Whether she has a father or not, she is a woman and women have natural desire to have sex as sex is a natural thing and all the humans want to experience it. Your guilt natural has to do with your upbringing.