I happen to know North Park as I own a couple of properties there.
Newbie, I presume your friend is happy where he lives since he got that “amazing” pre-construction deal. His adding a bathroom won’t do a thing to the comps unless he sells and “increases” values all around (even if he could influence values).
He bought for $370k so with the 10K bathroom and the cost of ownership, I suspect he would want $400k (or probably $450k to $475k for being smart and lucky to get that “amazing” deal).
You can get a much larger unit for less money in a better location on Illinois, closer to Hillcrest. What would a potential buyer choose?