These cases are tricky,
It may well be spigelian but also consider that it might be a ventral hernia through the linea alba where the linea alba is very wide due to the abdominal wall musculature weakness.
Ventral hernias through the linea alba are much more common than spigelian so I would bet on this one first . The fact that it is not in the midline is fine for this type of patient.
CT scan of the abdomen will settle the argument if you have any doubt.
Steve