It should work but the default sonar has very short range.  I haven't paid attention to Sonar-FF in a few months so someone can correct me if something has changed.

A while ago I tested a sonarman on neutral ships.  I assume because they are not the "same nation,"  I saw a severe penalty in range compared to using it on its actual nation ship.

The base range for a Sonar op is 9.25.   A DD or FF  without sonar should be
less then this. 

From experience I know there are nation weapon limitations on neut ships
so I would suspect that mako is right. There is a limitation compared to
using it on a nation ship.     I could not guess at the detection distance. 

I've used a DD1 as an AA escort with high lvl neuts. No sonar op on board and it spots
submerged subs at a decent distance.    However, I've used an FFY for sub hunting with no sonar and it seems to me that I spot subs closer then the DD1.   So, perhaps the spotting range for frigates is a little less then a DD.   

Never seen this mentioned anywhere so this is only a theory.     

Try a DD1 escort and see.                                       

Neutral ships have the worst maximum sonar range.Just use a nation ship and you should have much better range. ( assuming your sonarman is decent).