Forshayer , this case is not discussed in MB itself, but is in parentheses on the mechaber in seif 5, citing avudraham. The Shaar hatziun there cites prishah and pri megadim. I looked at the pri negadim, who says thar a person davening hoich counts for both him and the tzibur, and he says thet the person should say elokai netzur quietly afterwards…it would seem that the tzibur would have to wait for him to say it. I don’t have avudraham on me. After this I saw the magen avrohom (13) which seems to say that such a person needs to take 3 steps back to take his leave of the Shechina for his own prayers(since he didn’t daven shtil) “kegon elokai netzur,”….take from that what you will.
So your friend it seems would be yotzei everything, but he must take 3 steps back after the hoich.
However as for yourself…since you davened shtill as a sha’tz, there’s a good chance you can’t be yotzei if you don’t daven hoich afterwards, since the whole reason you’re davening shtil to begin with is to prepare for hoich.