That likely means:

No one could blame him for not being properly attired for his career

Especially as this sentence is following that one:

…, il portait une delicate et souple cotte de mailles noire qui miroitait doucement par-dessus de coquets entrelacs de laine noire et de cuir bouilli.

Here is “Apprêter” definition from the TLFi

APPRÊTER, verbe trans.
I. Emploi trans. Préparer.
A. [Le compl. désigne une pers., ou un de ses attributs] Préparer
attentivement en vue d'une décision, d'un événement imminent;
spéc., arranger soigneusement la toilette de quelqu'un :
    1. Le dimanche, maman m'apprêtait pour aller à la messe. J'avais
       de très beaux gants mais des souliers qui me blessaient.
       CHARDONNE, Romanesques, 1937, p. 123.

Edit: This text is actually a translation from Georges R.R Martin “Games of Thrones”. The original text is:

He wore black leather boots, black woolen pants, black moleskin
gloves, and a fine supple coat of gleaming black ringmail over layers
of black wool and boiled leather. Ser Waymar had been a Sworn Brother
of the Night’s Watch for less than half a year, but no one could say
he had not prepared for his vocation. At least insofar as his wardrobe
was concerned.

“s’apprêter” is to make oneself ready (prêt). So my suggestion would be something along to line:

No one could blame him for not having himself made ready for his calling.