Have your friend check out the resources here: https://www.healingwell.com/community/default.aspx?f=45&m=4097633
You didn’t say when your friend had his surgery. If he is in the early stages of recovery, swallowing can be very challenging. I was on liquids the first day, then moved to “anything that can be chewed to a liquid”. As SharonZ said, there is a lot of variety in surgeons’ preferred diet post-op. At some point during the first week or two I had a lot of mucus as a reaction to any swallowing. Your friend has had major surgery, and the mucous is a reaction to the irritation. My early recovery journal can be found among the other resources at the link above.
Tell your friend to try to relax into recovery. His/her surgeon can be called, and that can provide some comfort.Best wishes!