A penis has to actually enter the vagina for pregnancy to occur.
Sperm aren't magic or super-strength little cells. They can't just jump on in.

thanks a lot for replying .. i really hope they won't jump in .. but is there any outside chance ? should she take the morning care pill for that ..

No problem. Even if there were sperm on her panties, they can't really pass through and travel all the way to her fallopian tubes, it's pretty impossible. I wouldn't advise her to take the morning after pill because it's very, very, very, very unlikely that a pregnancy will result from this. Like I said, your penis has to actually enter her vagina. The morning after pill will throw her body into making her feel like she is pregnant and has some pretty harsh side effects. From personal experience, the pills made me vomit constantly for 3 days (They come in 2 pills), bleed for a week, and just make you feel awful! Honestly, I wouldn't worry about it.
Google it.