It's absolutely possible, though I'm certain that any support or help would be appreciated, especially if they're her first two kids. I have two kids that are a little closer together than that, and managed to take care of them just fine by myself. It was tough, but most mothers know how to push and sacrifice when their children need them. For me, it was also a plus that they were my second and third children -- my oldest is 5 years older than the middle child. Not only did the oldest give me a good foundation for what to expect, he was able to do a lot of things by himself by time they were born (i.e. go to the bathroom, get himself dressed, get in the car, comprehend instructions), so the workload stayed relatively manageable.
As for good mentally...it's never good mentally for someone to be without help and a support system. She may have some issues, especially if she has postpartum depression. It's not a total mental killer, and doesn't mean she can't take care of them, but it's extremely taxing. When I was at that point, I felt like I would give my entire world just for someone to watch the kids long enough for me to take a nap and a shower. If she turns down help, I'd take that as a sign that she doesn't feel in need of it right now, so no issue.