My friend watches my baby during the week. She has the STD herpes. Should I be concerned that my baby will contract it?

Good Hygiene. As long as the babysitter follows good hygiene by washing their hands after going to the bathroom and avoids kissing the baby - especially when they have an active cold sore, there is minimal to no risk of transmission of the herpes virus.
No worries! Genital herpes is never transmitted except by sex. Your baby will not be at risk from care by your friend. Don't worry about it. You're lucky to have such a good friend!