As mentioned, it all depends on what he would like to do with his character. Solo wise, D1 does help by providing more on-demand damage. I do agree with the lack of damage spells with Diev3, but TS is planning to go PD and will gain Incinerate. Also, his build has already has Zaibas for filler.
Diev3 offers utility, in addition to damage. Owls will last 25s more each, which is actually a big deal.
For utility, Ausrine is a refreshable 18s invul that lasts 30s. Aus > 18s up > Aus again for 16s (dance again when 10s is up) > Aus again before statue expires for 14s> = 42s of invul.
Ausrine has actually been working fine in dungeons from personal experience, and bugged skills get fixed anyway. Rotating SZ>Ausrine>SZ allows the Diev to tank bosses without budging for extended periods of time (usually over 1min non-stop, before having a short down time).
Both are valid choices for TS, depending on what he wants to focus on. However, it was important to make clear to him that Druid1 does not help PD in any direct way.