If Dhelmise does "Giga Drain" and the Defending Pokemon only has 10 HP left, does Dhelmise still heal itself for 30 damage?
Yes, because the 30 damage is still done, even though it is more than enough to KO the Defending Pokemon.
Source: Celestial Storm FAQ (2018-07-20), TPCi Rules Team (2018-08-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM7_22