99% of the time, the answer to this is yes, you can get the Freeview Satellite service from a Sky dish. Assuming the dish is still aligned correctly, and that the (intact) cabling goes to where you wish to have your Set-Top-Box (STB) it should be fine.
The 1% of the time when it might not work is where the dish is particularly old, and it has one of the early California Amps LNBs installed (The LNB is at the end of the arm where the cable terminates into). Many of these LNBs do not function at the polarity required by Freeview and will need to be replaced.
It is also possible that the dish is no longer aligned to the correct satellite – either it has moved in high winds, or else someone has re-aligned it to another satellite.
There are predominantly two different LNB Local Oscillator (LO) frequencies in use. The two used by Sky are 11300MHz for all older single LNBs (the whitish side of the LNB that faces the dish is round), and 10750MHz for the newer dual LNBs (oval shaped face). If you’re using a non-Freeview approved STB then you may need to change this setting in order to get it working. The Freeview approved decoders will detect this setting when first setup.