I DO understand the mentality your friend has.
to be frankly honest - it was the SAME mentality I had, when I had the same problem at that age. When you have a child THAT big, as some pp's have pointed out, you get in this mentality of "it doesn't matter, he'll just keep growing this fast and it would be a "wasted" expense, since it will only buy us a couple more weeks at most..."
I know it sounds silly now, but back then - it made COMPLETE sense to me. Or course, I did not know about the scenera, the only seats I knew that RF past 30 pounds were the Britax ones, which go to 33 pounds, and when your 9 month old is 31 pounds, chucking out $200 that you absolutely do not have for a seat that is going to gain you TWO POUNDS....can seem ridiculous. Even getting 4 more pounds out of a scenera for much less money can seem pointless.....
Anyway, of COURSE I think she should get a scenera, but all I"m saying is - if you haven't bveen there, it can be really hard to understand where her thoughts are coming from......when your 9 month old is in **2T** clothes, and you have a bunch of 12-24month clothes you never even worse because your child grew so quickly they went right from 6 -12months to 2T clothes due to their extreme girth........it is REALLY hard to believe that your child won't just keep growing.
The best thing I can say is...in a couple weeks, see if he has gained any more weight..if he is starting to plateau, which *IS* likely, then maybe she will be more open to discussion. Keep trying. I agree, maybe press the point that getting a better seat now will decrease costs later - a safeseat 2 isn't cheap..she could get something like a Radian for similar money......