If the dog is aggressive to other dogs, I definitely would not recommending letting her loose with your other dogs. How is the boxer with other dogs normally? Sometimes when a dog is under high stress, fear aggression can be the result of that. If the boxer just doesn't like other dogs period, keep her separate from yours. Walk her separately, take her outside alone and play with her alone. From what I understand, you're hanging onto the dog temporarily for a few months, so it's not 100% necessary for her to join your little dog pack.
However, if the dog is normally dog friendly (talk to your friend about this) but seems to be lashing out, then that's a different story. Again, I would not recommend just letting her loose with your dogs. Unless you want a dog fight on your hands.
Here's an article about how to introduce a new dog into a home with other dogs:
http://leerburg.com/introducingdogs.htm
If you browse the site, he also has other articles about pack dynamics that I absolutely recommend checking out. He also has an article on how to deal with dominant and/or aggressive dogs. But to be honest, I think it would be safest and probably the least stressful to just keep her separated from the other dogs while she's with you.
Hope this helps, and best of luck.