Background I am in a relationship with this girl who came to do her master's degree to my university for one year. We met in the Russian society and hence have mutual acquaintances from there. Since October, she got closer than with others, with a girl from the society(As in she became good acquaintance with her and they talk regularly). This girl is also my good acquaintance.
Case Me and my girlfriend are planning to go on a trip to Paris, for 4 days, during our spring break. The 4 days includes a day which is the birthday of my girlfriend. Her friend, which has her sister working in Paris, said that she would like to come and visit her. But she also said that she would like to come and hang out with us on my gf's birthday. So, the problem here is, we don't want to spend one of our four days in Paris with her. Her and I both agreed that although we both really like her, we would like the trip to be exclusively for the both of us only .
We talked about this with my gf and she said that she will tell this to her friend. (The girl is way closer to my gf than I am to her. We would not hang out at all if it wasn't to the connection of my gf to her.) So she doesn't know really how to say this without being rude or hurting their relationship. She said that she's scared bringing up this topic as she'd feel really awkward telling it.
So my question is: How does she tell her friend that we both would not like her at Paris with us, without being rude?
Also, since I don't have problems telling her friend that we would not like her at out trip, I told her that if she doesn't have the courage to do it until the 15th of February (our trip is 1st April) , then I will tell her myself. Although it would be way better IMO for her to tell her friend herself. She agreed to my condition, although she said that she will tell her, but is still not doing it...