I'm not too sure about that either, especially since the thing was also straining really hard to bite him at the same time. Zombies can multitask? Who knew?
Well I think I would struggle to rip open a person's chest more likely to break my fingers trying. Its a real cartoony death thats not needed in this show.
Remember though that we are talking about a zombie. If it broke its fingers ripping him open, it wouldn't stop to complain, it'd keep going regardless.
Personally i didn't think suspension of disbelief was needed at all. Humans have ripped open stomachs with sharpened fingernails before.
In Ancient Greece, one of the Olympic events was for two competitors to alternate hitting each other with a single blow, the first to incapacitate the other won.
One such competitor is historically noted to have sharpened his finger nails, struck his opponents belly with his fingers extended, cut open their stomach, then reached across to pull out their intestines.
He was of course disqualified, although not for the sharpened nails trick, but because the disembowelling counted as two hits.
With enough pressure and/or broken, jagged nails the zombie would be more than capable enough of tearing open Dale's stomach.