I'd say your friend has an inappropriately literal reading of the rule. "Insert Clip" is a colloquial landmine as Shadowrun and real life disagree on what a "clip" is. But more to the point of the flaw in your friend's reasoning:
The Insert Clip simple action (pg. 165, SR5) covers loading ALL ammo types, not just (c) ammo. By your friend's reasoning, if Insert Clip doesn't cover (b) ammo, then there's no action possible to reload them.
except you have to go to page 163 for the list of actions for reloading a fire arm... There is even a nice little table
Inserting a Clip (better known as a detachable Magazine) is the ONLY reloading action that is a simple action. Of course, it also takes a simple action to remove the expended clip first... so... well, other than notching a new arrow for bows.
I don't have the street lethal book (one of the few I missed) and I am not sure that the nimble fingers power actually does so I can not comment on what it allows an adept to do... I'll see if I can find it.
EDIT:
Found it.
NIMBLE FINGERS
COST: 0.25 PP
Adepts with this ability have exceptional manual dexterity.
In game terms, this grants +1 die to Palming
Tests, Performance Tests where the adept plays a musical
instruments that requires the use of fingers, or
any tests where small tools or items are used (gamemaster’s
discretion). Additionally, the Simple Actions
Change Gun Mode, Insert Clip, Remove Clip, and Use
Simple Device are now considered Free Actions.
It calls out that the following are considered "free actions" for an adept with Nimble fingers: Change Gun Mode, Insert Clip, Remove Clip, and Use
Simple Device.
Other then that, you get a +1 DP for palming, musical play..
and "test where small tools or items are used.." so many build/repair skills as well by my guess.
BUT there is a giant GM discretion listed right there... so it falls under the old catch all of "IF the GM agrees"
Depending on the Cavalier Champion's feed mechanics (I don't know what it is as I don't have that book.) you might be able to argue for a reduction to a simple action instead of a complex action for reloading....