Motion is a difficult thing to properly portray in a static medium, and especially so with a subject, or subject type, with which one has very little experience and is trying to improve on anyway.

Telling an artist flat out that they cannot draw motion, in general but especially when they’re trying to get a body type down, doesn’t help them.  It makes them feel like they’re failing at what they’re trying to do, and often discourages them from trying.

Don’t tell an artist that they can’t draw something.  Tell them that they need to work on drawing that thing.  Tell them that that thing needs work, and offer suggestions on how to do it.  That’s criticism, and it’s helpful.  Telling an artist that they can’t do something, be it motion, expressions, depth, whatever, isn’t criticism, it’s discouragement whether it’s intended or not.

Most importantly, don’t criticize an artist that’s not looking for criticism.  If they don’t ask, then oftentimes they don’t want it, and won’t take it if it’s offered.  If you’re convinced that they need it, and just haven’t openly admitted to it, then ask before you give criticism.