That depends on the kind of filter they put in.Good for them.....
But seriously,if you found out there was a problem with your water and there was a filter available to fix it......why wouldnt it have been done already by the homeowner?
Filters won't do the job according to some.
I trust plumbers know which to use.
Are you sure about that ? Would you let your children drink lead contaminated water based on the say so of a plumber????? I wouldn't.
I would have it tested after filtering before I did.