I'm not entirely clear about R's relationship with the mother of the lost baby? And the baby? Was the baby planned or was it an accidental pregnancy?
I reckon these things matter. If R is dealing with grieving the loss of a much wanted baby and the loss of a relationship with the baby's mum, then this might well not be the time to be making big decisions like moving a distance from his home and in with a friend and husband, hoping to have a relationship with the friend.
Did R want to move in with you guys before the loss of the baby? That might also give you some information.
Grief is hard to go through. My Dad died at the start of last year and one of the loves of my life died at the end of the year. The process is painful, frightening and utterly horrible to go through. I feel as though my world has been turned upside down and life is a struggle right now.
I suspect that the offer of moving away, starting a new relationship and being looked after by somebody I care for would be very appealing to me right now if it were to come up. The comfort and support would be very appealing. I'm equally as sure that it would be a bad idea for me to start a new relationship of any seriousness right now (bad for me and bad for anybody I got into a relationship with). My emotional capacity just isn't there just now.
If R is in the same place as I am, I'd wait a good while - roughly a year or so - before making those sorts of decisions.
If, however, the baby wasn't planned, R didn't want to be with the mum and is feeling enormous relief to be free from the situation, then I can't see why him moving to be with you guys would be a problem.