Hey. Why does my friend’s beer continue to have an apply smell/flavor? He’s made several beers in the past with similar results. The last beer, an oatmeal stout, he brewed with me at my house. He then transported the cooled wort back to his house for fermentation and later carbonation. I read that acetaldehyde can cause it???
Acettaldehyde will be a very sharp green apple sort of aroma/flavor - think jolly rancher. Usually, this is from trying to push the beer too fast, before the yeast have a chance to clean things up.
A more sort of sweet, red apple flavor/aroma is usually a fruity ester produced by yeast when they’re stressed. Normally for many brewers, that’s from not keeping the wort cool enough during ferment.
Tell him to leave his beer on the yeas for two weeks rather than transfering to secondary after a week.
My very first brew had apple cider type flavor/ aroma to it. I didn’t do much to control the fermenting temps, which got into 80s. Since that brew, I’ve paid close attention to my fermenting temps, keeping them in the low-mid 60s. I haven’t had the “apple” issue since, so I’m guessing with my example, I let my fermentation temps get too high.
High temps produces the nastier esters if any. More like fusel alcohols. Solventy- think nail-polish.
Cidery can be from bad yeast usually or incomplete fermentation. My neighbor drinks my beer and says “sweet-potato” but I realize he calls anything stronger than bud-lite that. 
He is ESL so that s his way of saying caramel.
Anyway it sent me chasing an acetaldehyde ghost though I did detect pumpkin in Windsor yeast and WLP029 especially at the third repitch on an ordinary Kolsch. This was at a month at 63F. I can only say yeast health was the primary factor to the problem.
Ya know, looking back on my notes for my first brew, I did only let it go 14 days before I bottled. So the cidery issue probably was from incomplete fermentation. No exploding bottles, but I did have some high carbonation to go along with the cidery issue.