First off, to answer your very first question: YES, it's absolutely ok for a non-parent to express concerns to a parent about a child's wellbeing. Some parents may not respect your opinion because you're a non-parent, but it is always always always ok for a CARING person to offer words of concern and support to someone they care about, regardless of who is who and what is what.
As for the specifics of your situation, I guess I would go at it from a whole different angle. Have you had any frank conversations with the dad about how he's feeling in general? How he's doing, does he feel he has the supports he needs, is he worried about anything? And then listen, since it sounds like you were friends before all this happened.
Then in another conversation, I'd tell the father what you've observed re: the son's time in video games, and maybe tell the dad that you're concerned because you know on the one hand that they're fun and that he seems to be having a good time playing them, but on the other hand sometimes being as immersed as the son is in the games can delay processing the major, monumental loss he's just experienced in losing his mom.
I'm not saying distraction and diversion isn't a totally appropriate thing to crave and have in this case - it is. But there IS such a thing as too much. And while you're not the person who gets to decide where the line is, as someone who cares about the son and the father, you DO get to offer some thoughts on trying to get the son to cut down on them, and also maybe going back to school (a new school), since socializing with kids his own age may also help him to process his loss and channel his energy into more productive things.
I lost my mom at a very young age, and while video games weren't really an option then I can definitely relate to the desire to immerse yourself in something that takes your mind off such a huge, giant life change and loss. But I'm SO GRATEFUL that my dad kept me focused on school and I stayed in close touch with my friends and tried to keep doing the things I'd done before.
It's totally understandable that dad is grieving and maybe he was a gamer to the point of contention in the marriage because he's got other things going on that gaming provides an escape from. But the truth is the pain and the hurt and the fears and anything else negative does NOT go away with more gaming... it just gets shelved in some ways but it's always there and it's always having an impact.
Ask your friend/the dad what he hopes for for his son, whether even in the midst of this big loss he's thought about what the best path is for his son to get the best life, and whether he'd consider helping his son to focus on some other activities as a way of giving him other channels to both learn and develop as well as process the loss.
Basically I'm mainly suggesting you talk to the dad, first about how he's doing himself, and then about how you know they're majorly grieving but you're also worried about the son not socializing with kids his age and going to school since those can be important ways for him to develop skills and behaviors that he's going to need as life goes along.
Last thing, maybe ask if you can take the son somewhere where a group of kids will be, and invite the friend and her kid who you said dad doesn't talk to. I'm not saying hide the friend from dad, but maybe just make it a group event and if dad asks who's coming, you tell him, but otherwise it's just a day in the park with a bunch of kids.... if the son really enjoys seeing that friend again, then mention it to dad and ask if they can have a regular play date.
It's all about checking in with dad on a lot of things, but mainly acknowledging how incredibly difficult this all must be and asking how he's doing.