The ITF Pro Circuit regulations (currently 2018 version, but effectively the same) include a code of conduct beginning on page 95.
Viviani's action violates part I, Abuse of balls in Article IV On-Site Offences as it clearly constitutes
... hitting a ball with negligent disregard of the consequences.
As it occurred during a match, it also carries a penalty from part R, Point Penalty Schedule, which progress from warning to point penalty to match penalty for third and subsequent offences. Under part S, Defaults the ITF Supervisor may also award an immediate default for any single offence under the code of conduct or after the third or a subsequent penalty under part R.
Viviani was given an immediate default under these parts of Article IV.
Given that an identical incident occurred in the men's tournament in the same year and was apparently not penalised at all, and that Viviani's offence was during the second set within previous offences in the match or the tournament at all, it is a fair statement that this penalty was unusually harsh in the circumstances.