2 internal GW 2014 users are on the same received email.
User1 received the email in HTML
This rather looks like two different messages coming in. At least at the GWIA level, as there are clearly two different messages files. It still might the the case that something (e.g. an entity scanning for viruses) splits one message into separate ones (for every receipient), but from GWIA's perspective these seem to be independent mails. And one of them apparently got garbled. This happens now and then if something "analyzes" and further on reconstructs a message. Checking mime.822 files might shine a brighter light on it.