by GamerAtom
the wording for the pet says it comes into play and THEN you place the pet marker on it and it gains the pet bonuses.Based on that, imo, when it first comes into play, it is not your pet as you havent put the pet marker on it. At this point, deathlock comes into effect. Then, as per the pet rules on the beastmaster card, you put the pet token on it making it your pet and gaining the bonuses. since deathlock is already in effect, it can not GAIN the +3 life.
So, my vote based on the wording and sequences of play based on that timing, is that your wife was right.