Another piece of Landorian Latin from Dry Sticks Fagoted. I had several goes at this: it's a little tricky to get right. 'Dolendus' is the gerund of 'deleo', which means both 'physical pain, hurt, suffering' and 'mental pain, grief, lamenting' [update: my mistake. Phil, in the comments, is right: this is the Gerundive, not the Gerund--'to be lamented', 'lamentable']. 'Dolentur', in the first line, means 'with pain or sorrow, painfully', and the repetition of terminology is hard to recapture in English. The sense, broadly, is: 'Unhappiness itself is when a man unhappily says ['dixerit' is third-person singular perfect active subjunctive of dīcō, 'I say', and the subjunctive is famously tricky to put across in English] he had once been a friend, now he is unworthy of that name.'
Dolendus ille qui dolenter dixerit
Erat olim amicus, esse nunc indignus est.
The saddest thing is that man who'll sadly say
He who'd been a friend once is unworthy today.
Suffering itself: the man who will sadly say
He was my friend once, but he is not so today.
Grievousness itself is he who'll say in grief
His passing time as my true friend: it was too brief.
Or maybe without the clunking rhyme:
Still not there. Hmm.
The most painful thing: he who's pained to say:
He had been my friend once; now he's unworthy.