Air licking should be checked out by a vet maybe on the next visit. It could be caused by a medical problem like a gum or tooth problem. It is often classified as an obsessive compulsive behaviour and there's debate on whether this is neurological or behavioural; but neurological problems can cause the behaviour too (eg SM). It is often seen as stress, anxiety or boredom-related as well. It may be no big deal at all but should be discussed with a vet and a vet may suggest a referral to a neurologist. It is also worth watching for any further developments and to have your friend be familiar with SM (ww.smcavalier.com). Yorkies are one of the breeds seen most often after cavaliers with this condition and air licking can be one sign. On the basis of mild air licking alone I wouldn't be too concerned (but would still discuss this with a vet) -- but it is really important for people to be aware of what might signal a worsening in the future or might be developing symptoms that should then be discussed again with a vet or neurologist.
It isn't an uncommon behaviour though and often doesn't seem to have any particular cause or be very distracting for the dog -- probably bothers owners more.
In general: ANY time a dog starts doing something that isn't normal, the first stop should be a vet. Eliminate a medical problem first, then go on to address any issue with other professional advice, such as a qualified trainer. On stopping this behaviour -- if it is medical or neurological, she will probably not be able to. If it is an obsessive behaviour, it's also hard to stop. Trying to stop it typically is counterproductive -- by fussing over the dog the dog simply gets *rewarded* -- with attention for the unwanted behaviour -- so it is encouraged to do the unwanted behaviour more frequently. It is generally better to ignore the behaviour. And have a ready supply of nice chew toys for what may well be a bored dog... eg a stuffed kong or nice chew.
If I had a dog doing this, I would have a nice chew toy always at the ready to redirect the dog's attention.
NB -- it is always frustrating to trainers when owners say things like 'my dog is just trying to get attention...". Why might this be? People are more ready to blame the dog than take a look at themselves and what they expect from their dog. This type of comment almost always suggests the dog truly isn't getting enough attention to start with! -- and most dogs do not get the time and attention from owners that they really need. The old trainer adage is true:
a tired dog is a good dog. If a dog is starting unwanted behaviours 'for attention' I'd turn the question back to the owner: what do you *actively* do with your dog each day? At least one 45-60 minute walk? A couple of fun mini obedience sessions each day? Fetch? Games? Agility? Does the dog have some great chew toys? Or is the dog expected to just sit around quietly every day, maybe getting a brief 15 minute walk and then that's it? Imagine how crazy a child would get, expected to sit quietly in a house all day! We would expect some challenging behaviour... and signs of boredom... maybe a destructive child trying to find something to keep himself entertained. Then we are surprised when dogs do this.