Identifying the molecular structure of compound with sum formula C8H14O3 formula using NMR and IR spectrometry data
Compare the quartet at $\pu{4.2ppm}$ and the singlet at $\pu{3.5ppm}$. These quite obviously would, in your friend’s structure, correspond to the protons on either side of the ether-oxygen. However, we should expect the methylene singlet to be more deshielded than the ethyl group’s methylene: It is squashed between an electron-withdrawing $\ce{OR}$ and an electron-withdrawing $\ce{C=O}$. The ethyl group’s methylene is between an electron-withdrawing $\ce{OR}$ and a mostly electron-neutral $\ce{CH3}$ group. Therefore, the order of peaks we observe in the spectrum does not make sense when we consider your friend’s proposal.
Thereafter, one might also argue that a quartet at $\pu{4.2ppm}$ is so obviously an ethyl ester as likely every organic chemist has had way to many NMR spectra after a column that contained a peak exactly there. An ethyl-ether is more shielded (diethyl ether’s methylene proton has a chemical shift of $\pu{3.5ppm}$.