Well...this is really based on chance. You said you've "heard" that the antibiotics "can" affect birth control. Where did you hear this? Was it a reliable source? Did it give statistics, reasons, or studies as references?
If you don't know about any of this, you should definately look it up and ask your/her doctor if the antibiotic will affect the birth control (and if the doctor says it "might", ask her what the chances are).
Then you said "the condom came off inside her before I came". Well....did you come inside her after the condom came off or did you pull out? While there *are* fluids released into the condom before you come, there's a much smaller chance of her becoming pregnant if you pulled out once you realized what happened. Additionally, if you used a condom or lubricant that had spermicide on it, there's a chance that the fluids in the condom were killed off when you lost the condom. The chances of it killing everything, though, are much less likely if you didn't pull out.
Do you notice a trend here? "Chance", "Might", "Maybe", "If". There is no clear-cut answer. The best thing to do is wait a few weeks, see if she gets her period, and if it's late, get her a pregnancy test.
And while you wait, it's good to call up your doctor and ask about the drugs mixing. Oh yeah...and I hope you got that condom out. Because if it's still in there, you might want to do something about that too
