How rude I would consider this to be depends on a lot of factors, including what kind of event it is, how close the friendship is, and whether the person promised to play any specific significant role in the event, rather than just being one of the crowd.
Also, there's the problem that friendship, in general, is a completely informal thing in (at least most of) contemporary Western culture. Thus there is often a lack of clearly defined mutual obligations between even the closest of friends. This can lead to a lot of misunderstandings and feelings of abandonment.
Maybe the modern Western world needs a revival of more formal, structured kinds of friendship, e.g. fraternal organizations?
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