If a doctor, trainer, etc., commits shirk, like an Hindu, is it forbidden to see them?
No it's not forbidden. You're only going there for medical treatment and nothing religious.
Prophet Mohamed and Abu Bakr aSadiq used the help of a pagan as a guide to find their way to Medina from Mekka.
The Prophet and Abu Bakr employed a man from the tribe of Bani Ad-Dail and the tribe of Bani Abu bin Adi as a guide. He was an expert guide and he broke the oath contract which he had to abide by with the tribe of Al-Asi bin Wail and he was on the religion of Quraish pagans. The Prophet and Abu Bakr had confidence in him and gave him their riding camels and told him to bring them to the Cave of Thaur after three days. So, he brought them their two riding camels after three days and both of them set out accompanied by Amir bin Fuhaira and the Dili guide who guided them below Mecca along the road leading to the sea-shore.