FMC's mom writes note to lover saying she's accepted a duke's proposal? FMC thinks MMC left her

I believe it's a second book in series and the MMC is the brother's friend. The FMC and MMC have an secret romance when they're young (18-20) and the mother (who is not a good person) finds out and pretends to write a letter from her daughter to the MMC saying she's accepted a proposal from a duke. The MMC is maybe a second son at the time but later comes back years later broke but now the heir or Earl/Viscount. They're reunited (I think) we he comes back to England and is visiting the FMC brother. They're really bitter towards eachother because both don't know what happened.