In short: it depends on the drug used.
Each and every drug is metabolized differently than other drugs. Some are metabolized by the liver by CYP450 enzymes, others are metabolized in other parts of the body by different enzymes, some break down naturally into other drugs, etc. Even one single drug can be broken down in a multitude of ways. Regarding psychedelics as a whole, this question cannot be definitely answered. Which psychedelic(s) in particular are you concerned about and what form are they dosed in (i.e. are they dosed as pure refined chemicals or are they administered via a natural medium such as a mushroom or tea made directly from natural products)? Ultimately your question rests on these pretenses: the kind of drug used and the medium it is administered in. If the drug is metabolized hepatically (in liver) the next question should be which enzymes and are those enzyme levels/functioning impaired by the presence of hep c?
Also, another thing to think of would be how advanced/controlled is the Hepatitis C? How well does their liver currently function? Are they seeing a Dr. for treatment of Hep C, and if so, is it well controlled and which medications are they currently taking? Some meds for Hep C may interfere with metabolic processes so keep that in mind as well. Also, a fairly well functioning liver with Hep C would be better able to metabolize drugs than a dysfunctional liver on its last leg. Yet again, this issue is a product of the circumstances surrounding it. Also, are there any kinds of drugs their doctor has specifically told them NOT to take since they have Hep C (certain kinds of drugs are metabolized via certain liver enzymes-if their Dr. told them which kinds of drugs not to take in particular it could be a sign that certain enzymes which also metabolize certain psychs may not be functioning normally and hence it wouldn't be safe to consume drug XYZ).
We cannot tell you for sure if it would be safe unless we knew which psychedelic in particular you were concerned with, the medium/ROA it was administered in and through, which meds the patient is currently taking for hep c, and how well their liver is currently functioning under current conditions.
I would be happy to answer any and all questions you may have; corpus callosum would also be a good person to PM with specifics as well.
Wishing you well,
-God