Girlfriend deleted message from another guy, what does this mean?

I've been dating my girlfriend for more than 14 months. We go to different schools so we dont see each other often. One of her "friends" that goes to her school has a car. She asked him for a ride to her home and he responded, "I'll give you a ride if you give me head while I drive". He knows that she has a boyfriend as she told me she tells him about me. She tells me that he's only joking, he also calls her cute and hits on her daily. Recently when i went back to see her i asked if i can see her messages with him and she says that she deleted their convo. What does this mean? She said she deleted the convo because she doesn't talk to him anymore and that she would be "embarrassed" if i read her texts with him. Why do you guys think she deleted her texts with him? What should I do in this situation?
Girlfriend deleted message from another guy, what does this mean?
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