My friend just recently had a baby 7 weeks ago. She had sex & she took the plan b pill just for precaution. Her period came on a week later but it only lasted for the 3 days. The first 2 days were heavy but the 3rd day wasn't. Could she be pregnant

Pregnancy is possible any time you have sex. Plan B is not 100% effective, so there is a chance that she got pregnant even after using the Plan B.

It is very dangerous to have two babies back to back, it's very hard on the body.

Why isn't she using another form, or two, of birth control? Plan B is not recommended as a form of contraception, it's not ideal for your health. It's only for accidents (forgot a pill, the condom broke or slipped off, etc), not as a form of birth control.