I don't recall ever seeing a snooker player help fetch a color during a frame in a refereed match. I suspect it happens sometimes if there is no ref. Pro players do sometimes fetch balls at the end of a frame but the frame will have been declared won at that point.... and not even attempt to help the ref with the balls, like in Snooker.
There is a story of an exception, though. I forget which book I read it in. It seems there was this well-known strict referee....
The match between Smith and Jones was down to the final black ball of the final frame with the score tied. The next score would end the match. Smith made a difficult cut along the rail, the black went down, and the ref began, "Match ...", and before he could finish Smith grabbed up the still-moving cue ball in triumph. The ref continued, "to Mr. Jones." Consternation and earnest discussion ensued, but the ruling stood.
As I recall, the story was related in a way that made it seem like it really had happened as opposed to a fabricated tale to warn against counting your chickens...