An 8 month old pregnant woman had had cs due to hbp bout 20 months ago and now she is 8month pregnant and her medical doctor another cs, is it safe?

Standard. It is standard practice. Vaginal delivery after csection is possible, but carries risk of uterine rupture. You doctor's advise sounds reasonable, if csection is done under regional anesthesia.
Check with OB/GYN MD. She can have a vaginal delivery after the prior c/s depending on her and baby's condition. But most of the time repeat c/section would be an option for an obstetrician in this situation. Mostly it depends on the skills and practice preferences. Repeat c/section has almost the same risks as a first one. Consult your ob/gyn.