"When in the judgment hall the words of denial had been spoken; when Peter's love and loyalty, awakened under the Saviour's glance of pity and love and sorrow, had sent him forth to the garden where Christ had wept and prayed; when his tears of remorse dropped upon the sod that had been moistened with the blood drops of His agony—then the Saviour's words, “I have prayed for thee: ... when thou art converted, strengthen thy brethren,” were a stay to his soul. Christ, though foreseeing his sin, had not abandoned him to despair. Ed 89.1"
"But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren."
EGW directly quotes and elaborates on the specific verse, Luke 22:32, in the context of Peter's denial and subsequent repentance.