Please read what @Iwant2move has posted above.
Your BIL has suffered a huge loss, the person he lived with, loved, held, had DC with has gone.
That time may be counted in months, but what if each hour without her feels like years to him?
What if the house seems empty, the days silent, the bed too wide, the To Do list too long & the need to be held, soothed, loved too great to let a potential love interest go by?
What if a new partner in his life allows him to smile, sleep, have something to get up for in the morning, plan, see a life worth living?
Do you really begrudge him that?