I won't go into specifics like with the majority of people that post here. Your friend doesn't really need fine tuning, they really just appear to be new to dps in general, so the advice will be more generalized.
First and foremost have them start hitting buttons at all times. There are several(a whole damn lot) of times when they aren't doing anything at all. I don't mean like .5seconds between shots, their are lot of 4+ second gaps of doing absolutely nothing. For example look at this graph from butcher-
https://www.warcraftlogs.com/reports...ce&view=events . All of the green bars are their casts, all the spaces with no bars is them going out for coffee or something, who the hell knows.
Second, have them get some form of addon (either TMW or WA) and copy some people's strings so that they know when important shots are available.
Once they do those 2 things, you should notice considerable improvement. After that come back and we can worry about specific things like DOT uptime, talents, and optimizing, but for right now they really just need to start with the very very basics.
Edit: I know you said you don't know much about hunters, so if you need help with WA or TMW strings send me a PM, I can give you my WA or create some for TMW(they are pretty easy and quick)