Re-entering HK visa-free on a US passport

e have a US citizen visiting us - she will be staying in Hong Kong for 90 days (max visa free period), then plans to leave for a week and then re-enter Hong Kong. I'm trying to find out if there are any restrictions on re-entry, e.g. does she need to be away for a minimum number of days, or is there a limit to the total number of days she can spend in HK in a given year, etc?

Any info appreciated!