Kind of a surprise that it was Wendy's mother's anonymous lover who was the kind voice of reason who helped smooth the way for the two of them to leave for boarding school. That's... not how that character is usually used in these kinds of stories. ><;
Was it? I thought maybe the husband was now back from the war . . . but maybe you're right.
Marly noticing that the man with Wendy's mother was different from who she remembered could work either way; either she remembers the wartime lover and is surprised to now see the husband, or she remembers the husband from before the war and is surprised to now see the lover. Lemme think . . . actually, I don't think Marly knew Wendy before the war and the lover. 'Cause, she first got to know Wendy when her mother was crying all the time, which would be during the war when she had a lover. So the first man she would have associated with Wendy's mother would be the lover. Which means if there's a different guy around now, it's either a different anonymous lover, or it's the husband, back from the war. Also, Wendy's story says "While he was gone for the war", which suggests he is no longer gone for the war.
So pretty sure guy stepping up is Wendy's returned father.