As long as you don't have preferred orientation then you can compare relative intensities of different phases in the same pattern to get a semi-quantitative phase ratio as long as you do the calculation correctly.

u can use reitveld refinmnt to determine relative ratio of Mg and MgO in the 2smples independently

I don't think he needs to do Reitveld refinement. Unless there is someone there who can teach him to use a Reitveld program correctly or he goes to a workshop, he is more likely to get a better result comparing integrated intensities taking into account the proper structure factors.

if it is in the powder form and you don't have a preferred orientation, you can qualitatively compare them. but quantitatively comparison could be inaccurate.  you should be able to tell that this sample has more of  a certain phase compared to the other.

Comparing relative intensities of phases is what the (semi)quantitative RIR method is based upon.  Most XRD books written after the mid-1980's should have a section on the RIR method.

A quick comparison of the two peaks, in the absence of preferred orientation (as previously mentioned) will give you an indication of the amount of MgO vs. Mg remaining in the system. 

In fact the Rietveld refinement is not necessary in this case. The RIR method can work very well. Pay attention to choose a peak corresponding to a single reflection,  without overlapping, otherwise the results can be wrong.