Understanding a feeling does not require endorsing every conclusion
You can recognise someone's disappointment without agreeing that everyone treated them unfairly. Saying that missing the invitation sounds painful acknowledges their experience. It leaves room to discuss what happened without requiring you to confirm an explanation you do not know is true.
Go deeper
Separate the experience from the explanation
A friend was not invited to a gathering and concludes that nobody values them. Their hurt is real, while the broad explanation may be uncertain.
Acknowledging the feeling does not require verifying the conclusion. You can recognise that being left out was painful without claiming to know what every organiser intended.
This distinction allows warmth and honesty in the same response. It also avoids forcing a choice between immediate disagreement and complete endorsement.
The conversation might later explore what is known, what remains unclear or whether the person wants to ask about it. The first acknowledgment can simply name the experience they are describing, without turning an uncertain interpretation into an established fact.