Answer:
His friends have fallen prey to the post hoc fallacy.
Step-by-step explanation:
A fallacy is an invalid reasoning which has no grounds but it can seem as a valid argument.
Post hoc comes from latin Post hoc ergo propter hoc, which literally translates to "after this, therefore because of this". This means that there is a belief that a certain event is the result of another event, that there is a relation between the two which can be completely unrelated.
In this case, Gino's girlfriend broke up with him right after he gets the tattoo, she might've not even known about it or had other reasons for breaking up with him. However, his friends think these two events are related and that the breakup is the result of the tattoo, thus falling prey to the post hoc fallacy.