Cynddwrg also means 'so bad'
Which means we only need to change the last bit.
'Maen nhw cynddrwg gyda'i gilydd'
If you were to unpick the phrase ...nhw ... â'i gilydd it would have translated as 'they each ... as his fellow', with the his/her being singular.
However, cilydd is no longer in use on its own in modern Welsh and the phrase has become a fixed one meaning 'they ... as each other'.