Thanks for posting your query at DoctorSpring.com
As such, some amount of mild spotting happens sometimes in pregnancy.
However, any kind of bleeding or spotting should be thoroughly looked into.
She should be having a low lying placenta or a placenta that bleeds easily.
A locally situated polyp could also bleed like that.
You should not panic, but yes, book an appointment with your obstetrician at the earliest.
Have an ultrasound scan done to assess fetal well being and document heart beat and to localize the placenta.
Avoid sex till you get an all - clear after the scan and internal exam.
All the best
Please feel free to discuss further.