I think it depends on how old the son is and what kind of help the mother has in mind. Is she think her son needs male companionship? Is he acting up? Does she think he needs to talk to someone outside his immediate family?
This question hits the heart for me, because my younger brother was the son was widow at sixteen years old. I'm sure losing his father at sixteen had its impact on a son (needless to say), but your question makes me think of how my mother was strong enough to "remain the parent", rather than allow herself to start to rely too much on either my brother or me (I was twenty-one and would do things around the house that my father had done) or my sister, who had a toddler and did not live nearby.
So, my immediate reaction to your question is that the best thing anyone could do for the son (assuming he's still a kid) would be to let him know that his mother is still strong and still the parent; and that he doesn't have to shoulder the burden of "being man of the house" because his father has died. His mother (and all single/widowed mothers of boys) needs to be solid enough, and sure enough, that she can muster up what it takes to be remain a strong guide and support for her son. She's got to be strong enough to shoulder her own and her son's grief, and strong enough not to give up being a whole parent to her son; because now he has only her. If he's not sure she can do it, she needs to support and reminding that she can. If she needs support from a counselor for herself, that may help her. If she needs back-up from an objective outsider that her son respects, then a counselor could offer that too.
From another angle (and having lost both of my parents), if you're a relative or friend; maybe doing something that would help the son get his mind of his grief for awhile would make a big difference for him. Sometimes a temporary break from grief can give the mind a little rest. You could do something with the son and/or with both of them, and maybe just share some pleasant conversation.
If the problem is that he's, for example, a teen who is acting up because he doesn't respect her; I think she's putting too much on you and really does need to see a counselor with her son. This is the best answer I can come up with, knowing so little about the son.