Is it possible he messed up something while installing the filter ?
Anything is possible. But there's a way to check.
With the filter dial valve in the Filter setting, take the cover off of the skimmer:
1. Is water moving in the skimmer?
2. Is water coming out of the return jet(s)?
If the answer to those questions is yes, the filter is plummed correctly.
What else I can do ?
You can provide more information.
Is this an above ground or in-ground pool.
If in-ground, is it a vinyl liner, a plaster/gunnite or a fiberglass pool?
If in-ground does it have a skimmer and a main drain?
How many gallons does it hold?
A complete set of chemical readings:
Combined Chlorine or Total Chlorine
Tell us if you took the readings yourself, (and if you did, did you use drop reagents or test strips) or if a pool store did the water testing.