。
Breaking his promise — he's irresponsible too.
Literal
Promise [obj-を] break-[exclam-とは] he [also-も] irresponsible [copula-だ].
The same criticism formulated with ~とは instead of ~なんて, giving a more measured/literary tone. The 彼も ('he too') implies the speaker has encountered similar irresponsibility elsewhere — 'he too, like others.' Together: 'to break a promise — he's irresponsible as well.'