NGS was done on MSI-H tumor. Tumor has 30 percent purity and was reported seperately by pathologist. Many point mutations identified-20 out of 125. One frameshift mutation mutant fraction is 35 percent. The rest of the mutations are 20-28 percent. Would the mutation with a mutant fraction of 35 percent be a homozygous mutation?
1) Your ability to infer this depends greatly on the depth of sequencing. It helps to picture each VAF with error bars on it, that grow smaller as your depth increases. In 30x sequence, 35% is absolutely not significantly different from 20-28%. In a 500x tumor, it probably is.
2) Without seeing your data, I'll assume that your tumor purity is somewhere around the median of the VAFs you describe. (20-28, so we'll call it 24%). That means you'd expect tumor purity to be double that (48%), and homozygous SNVs should appear around 48%. That doesn't seem to fit your frameshift VAF, so we can explore other hypotheses. What if there is copy number amplification of the mutant allele (3x CN). That would lead to a VAF of 1.5 times the baseline, which would be 36%. That seems to fit your point nicely, but you should go looking for corroborating evidence. What does CN inference tell you? What are the b-allele frequencies of germline SNPs in that region? etc.