Dare I go against the grain here..
Not all lone parents are dishonest about having a partner present in their homes. Just because you see a guy going in and out of the house but 'know' she's on LP, doesn't mean she hasn't notified the SW.
I know this is the case for my brother and his gf. A nosy neighbour of theirs threw that one at them one day, and my brother said with a grin "you do that, call the social welfare".
I think it's wrong for people to be claiming for SW if theres another income in the house on the sly, but I'd want to be 100% sure of it before I go lifting the phone. It's not right to be meddling in other peoples lives without having the proof.