Jo
One month is still a short time after the passing of a loved one. But perhaps you can mention on how much the loss of your friend and sister in law has affected you ,and how he must be really suffering from the loss of his beloved wife and Mum, the grief process is very personal some people can talk about it and others can't for quite a while,
I an sure the nights will be very long and lonely for him maybe ask about them.
A friend of mine lost her partner 3 months ago but at the start after her loss it was very emotional and lots of tears,the tears are still there but at least we are able to talk about how she feels and her emotions.Only you know when the time is right.Hope this helps
kj