Pipe Down has mastered many techniques with regards to the art of story telling. However, his senility prevents any understanding of tact. Will he be able to tell Rarity the truth about her parents without mortifying her? No, but only because he's too old for that.
Pipe Down regales the CMC with the story of how he earned his cutie mark.
Rarity doesn't want to know how her parents met. She'll hear the story anyway.