That would only protect you from common threats, which the highly specialiced trojans/keyloggers out for bitcoin exchange login data most definitely are not. The only way to confidently deny the presence of bitcoin malware is if you'd exclusively accessed exchanges via a fresh and clean VM.
It wouldn't be enough if the host OS wasn't "fresh and clean" too.
It would probably be enough if he used the host OS exclusively for accessing exchanges and the guest OS for other Internet-interacting stuff. Though it may still be sometimes possible to compromise the host OS from the guest OS (or other guests if they aren't properly isolated).