Such deaths are not only hard for the immediate family, but also difficult to explain to others, as they may quickly be judgmental.
No, it is not necessary at all for you to inform people of the cause of death, and you can just say "he/she passed away, please pray for him/her." And if they have a follow-up question, repeat that sentence.
As you know, lying is haram, and so you should not make something else up, but just rather feel confident that you do not need to tell people this kind of personal information.
If they care and are concerned, they would just leave it at that, and respect your feelings and privacy and just do dua for the marhoom/marhoomah.