Lot of variables here.
How do you know they only met 3 months ago, maybe they knew each other and only started dating 3 months ago.
Could have been an affair during the marriage and they kept it quiet until now.
Maybe the marriage wasn't a happy one for quite some time and even though I am sure he grieved at her passing, it could be less grieving at losing a wife, and more grieving for his children losing their mother.
Could be lots of things..........
I say too soon is different for different people. I know people happily married for years and only knew each other for a few weeks before getting married, and I also know people who are divorced that dated and lived together for a total of 12 YEARS before getting married.