Mechanic is justified to recommend a flush.
As long as they use a radiator flush or citric acid (weak food grade acid) it is good.
I have heard of some people using strong acid here....that is wrong.
Even with citric acid, you are supposed to run a solution of meetha soda to neutraluze the system after cleaning. (Meetha soda = carbonate)
If they do something silly, skip their service, flush it with plain water a few times and then use coolant.
Zic or pyro are good, replace coolant after 2 yeaes