I am somewhat *amazed* that an individual can have have three rental properties vacant simultaneously.
I think it would be well worth your while getting a description of your friend's properties to get your own feel for how good they are. Take a look at the properties even (if your friend permits), ask what he/she is charging for them (and why). Determine what kind of yield your friend is trying to achieve (easiest if you simply ask how much they are worth) and then determine if it's realistic based on comparative yields for the area. Your friend may have dropped the rent but it may have been high to begin with. It may have been too high because his yield expectation was too high compared to the remainder of the suburb/area. Yields have certainly dropped owing to the property boom (yields cannot keep up with growth fast enough) and possibly indirectly due to the First Home Owners Grant (FHOG).
Are your friend's properties all in the same area? Are they all of the same type of accommodation. Perhaps your friend's predicament is a good reason to have some degree of diversification in what properties you invest in (that's just a casual observation on my part).
Someone said to me a while back that if the vacancy rate is 5%, in theory *on average* you would need 20 properties to have one vacant. You can therefore understand my amazement (unless your friend owns 60 properties!). Or, you have a 1 in 20 chance of having a vacant property. Not fantastic odds.
But remember, that 5% vacancy rate includes all the *crap* property out there and all the *expensive* property, etc. Keep in mind that an "average" means there are some values below the average and some above. If you divide properties into different "bands" (by whatever criteria), some bands of properties will have lower vacancy rates and some will have higher vacancy rates within those bands. You can improve your odds (ie. lessen the chance of vacancy) by making your property more attractive to a tenant, whether that is by virtue of improvements, reduced rent, better location, perks in the lease agreement, etc.
For example, if you split property into bands like "units" and "houses" you may find the vacancy rates are different for each. If you split property into "high-rise apartments" and "other" you might the vacancy rates are different for each. If you split across suburbs you will get different vacancy rates in each suburb (you get the idea).
Having a property without a tenant would be scary, but only if you can't afford to service the debt whilst it is not tenanted. The question is how long such a situation might continue for if you have a reasonable property charging reasonable rent? You should be able to service the property for at least that period of time to remain comfortable and retain the "sleep at night" factor. Tenancy is one of the critical factors in the balance sheet of a property, yet all too often people accept it as a "given" (talking about property marketers, for one).