。
He's really fallen if he'd lie to me.
Literal
I [to-に] lie [object-を] tell [quotative-とは] he [also-も] fell thing is.
とは at the start of the sentence (following a verb phrase) marks the preceding clause as a quotation one is reacting to with surprise or judgment — 'to [do such a thing]!' 彼も落ちたもんだ = '[it's] that he too has fallen' — もんだ (もの+だ) is a sentence-final pattern expressing admiration, exclamation, or nostalgic/judgmental evaluation. A formal-feeling expression of disappointment.