Is the steady state of $k$ enough to show $Y$ grows at the rate $n+g$? To provide some context for the question: I was approached by a student needing help for a an empirical thesis based on Solow-Swan model. She had trouble solving the theoretical model she wanted to showcase in her work. She already put a lot of efforts and had a problem just with last steps of derivations so I helped her to solve simplified version of Solow-Swan model where:

$$ k^*= \left( \frac{s}{d+ n + g} \right)^{\frac{1}{1-\gamma}}$$

where $k$ is the capital per effective labor, $n$ population growth and $g$ technology growth and $d$ depreciation, and told her that this results implies that $K$ is growing at the rate $n+g$ which implies due to the assumption of constant returns that $Y$ grows at the same rate. Since $g$ and $n$ are exogenous the growth must be exogenous as well. 

But her supervisor told her that the above is not enough to show that economic growth is driven by exogenous factors and she has to expand her model.  

Hence my question is: was my statement incorrect or is her supervisor making mistake? If my advice was incorrect whats the missing link that unambiguously shows that economic growth depends on $n+g$?
Based on the steady state your production function is Cobb-Douglas.Taking logs and derivatives wrt time of $Y$, $\frac{Y}{L}$ and $\frac{Y}{AL}$ in the steady state yields the desired result: $K$ grows with $n+g$ on the BGP. 

It would be interesting to know what the supervisor's objection was.