My neighbor who lives in a mobile home shower stem bonnet broke (water won't shut off) plumber told her they are not allowed to replace the ring washers or stem wthey could lose their license that they have to replace the whole system with the new scald proof ones charging her $350.00 I just replaced my stem myself for $10 is this plumber telling the truth?

No.

You know what, maybe that guy should loose his license. What a bunch of baloney.