Dear Yamuna,

Some institutions such as JIPMER doesn't admit candidates who have already completed or are in the midst of completing an MD/MS. However, most colleges/institutions admit such students in second MD/MS, provided they have already completed one PG course. Having said this, some universities might also give admission to candidates already pursuing the PG Course. For the same, I suggest you check the university specific website. 

Any which way, the probability of this happening ain't so bleak. 

Furthermore, talking of Diploma, one of the major entrance tests for that is DNB PDCET. Like NEET PG, it is also conducted by NBE. Your friend can appear for the test and take admission to the relevant courses, ONLY IF the PG course one is pursuing has been completed. 

Hope this answers. All the best!