Wait, I was under the impression they are exes?
Like she broke up and he was all in denial like "I don't wanna break up!" and making a fuss over her moving on. Then wife suddenly died and he is being "no, we can't break up now, I need you!"
OP -- are you broken up? The timing is terrible... But if you broke up, you broke up.
In those shoes, I wouldn't want to give GF level support here. Makes emotional weirdness when you are trying to detach.
As a friend, sure -- attend the funeral, pay my respects, grieve myself for a person that was a part of my life, be kind to him and the adult kids, show compassion, make a casserole, walk the dog, etc in friend appropriate ways if the rest of the family does not see this as intrusion of their mourning period.
But getting sucked back into some weird rebound romance relationship thing... now?
You don't mess with fragile. Backing off to an emotionally appropriate place still seems better to me.
It would be unseemly in a monoship. For his wife to die and for him to start up with a new lover person before she's even cold.
Since your old thing ended, you have no thing now. It would be starting a new thing again if you got back together. I just can't see how it is prudent to start up a new thing at this time -- monoship or polyship -- in a time of emotional upheaval. Esp when in the past he was not great at polyshipping with you to begin with.