"She is wondering who he is."
Translation:Hun lurer på hvem han er.
"Hvem som" cannot be used together with personal pronouns (him/her/I). The "som" excludes the personal pronouns when the question word "hvem" is used.
"Jeg vet hvem (det var) som gjorde det. Det var han som gjorde det. Han gjorde det." I know who (it was that) did it. It was him who did it. He did it.