Idiom about forgetting about someone/something
One possibility is treated them like chopped liver. This is a widely used idiom, indicating that he treated them as though they were insignificant.
Another idiom could be wrote them off.
If the omission was really grave and could lead to serious consequences, one choice could be hung them out to dry.
More "standard" possibilities might include:
- showed no regard for them or disregarded them
- did not take them into consideration or did not consider them
- overlooked them
- left them out
- ignored them
As always, of course, context should determine your choice.
Your question: Someone provides something that only caters to half the group - what's an idiom for what he did to the other half by forgetting about them?
Perhaps he gave short shrift or didn't give a second thought to half the group. From the idiom component of The Free Dictionary:
give short shrift: To ignore, disregard, or exclude (someone or something); to give (someone or something) very little time or attention.
not give (something) a second thought: To not give some situation or decision pause for any hesitation, concern, or further consideration.