A question worth pondering - is it crank, or just alternative?
What's the difference? Cranks believe that the truth is being suppressed, that their work is particularly astounding, and that any criticism is an attempt to suppress them.
If the views are merely "alternative", then they're trying to explore possibilities, and that's actually an acceptable thing. Most new theories begin as "alternative", and the greatest discoveries happen when someone is viewing things from an unusual perspective.
The approach to helping your friend to be better is different depending on which category it falls into. True cranks need to discover their problem for themselves, which means you need to point them in the direction that will lead them to self-discovery of the problems. Alternative researchers are best helped through guidance - often the best approach here is to ask probing questions and offer possible ways to improve the work, rather than trying to get it discarded entirely.
To demonstrate the difference between these, consider General Relativity and theories that compete with it.
In the "alternative" category, you will find a broad array of alternatives, many of which have been tested and found to not match experiments, but some of which have come from quite well-known physicists and mathematicians. This is healthy - if General Relativity is going to be improved upon, it will be through development of alternatives.
Meanwhile, an example of a "crank" within this field is Autodynamics. The creator of this "theory" (a term I am using quite loosely, and unscientifically) claims that his first book on it was immediately blacklisted by the "National Atomic Energy Commission" of Argentina, and that a daily newspaper published an account of his theory, and was then threatened with being shut down if they published any more from him. To be clear, this is information provided on the official Autodynamics website (which I won't link to, here - if you want to view it, follow the above Wikipedia link, it's linked from there). Essentially, the claim is that Autodynamics didn't "win" against General Relativity because it's being suppressed.
It can, at times, be hard to tell the difference. The best way to tell, is that aspect of claimed suppression. Does your friend believe their work is being suppressed?