While having intercourse the condom fell half way down after i ejaculated. some semen may have come out of the condom. how high is the pregnancy chance? what can i do about it?

While pregnancy is possible, it is diminished given the information described. If your partner's period is delayed, get a serum HCG test to be certain about pregnancy.

Pregnancy is possible so your partner should take plan B if she does not want to get pregnant. Plan B works best if taken in the next morning but it may be still effective within 72 hours.