The answer depends on why she wants the other guy to move in with you. If it is monetary and having a 3rd roommate will allow you to move into a better/larger place or is this a good friend of hers who wants to move also. If money is the issue, if you can you may want to wait to make the move. If it is a good friend of hers you may want to explain to her that this is an opportunity for the two of you to spend more time together and having the other guy there would make you uncomfortable.
No matter what the reason is BE HONEST with her and tell her how you feel about the other guy moving in. She may not like it but it may be the best for your relationship.
Best of luck