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Her original plan was to use a convertible from birth (in fact, she has the True Fit Si already). I pointed out to her that I wasn't sure if the TF was a good fit for newborns and to consider that. Since you have that seat, have you tried it with a newborn? This is why I wanted to give her an infant seat as a gift, to make sure she has something that will fit a newborn and because she mentioned traveling by plane with the baby when he will be just a few months old. An infant seat will be way more convenient to travel with! Also, I think she will find it will be more convenient during our awful hot summers to have a seat in the house to put baby in rather than having to put him in a hot seat. I know you can throw a towel on the seat to try and keep it cool but honestly, when it is 115 degrees out in mid July, it won't matter!
I got my True Fit after my son turned 1 so I don't know if it would fit newborns well, but it does have short first straps and a nice padding system that lifts the baby up, and the headrest is removable for the littles (under 22 pounds and the red line) so that it can more easily fit reclined in more cars, so I think its ok for newborns (but obviously all newborns are different).
But I would NOT travel with that seat, it is a beast to move, so heavy and bulky! Once my husband manged to move it from my sisters to minivan to our car with my son in it without waking him but I would not do it on a regular basis.
Another thing for her to keep in mind if she plans on ERF is the weight limit. My son is a skinny minny so the 35 rf weight limit should last us until he's 6 if I was so inclined, but it may not be the best choice for chunkier babies, in fact I find it funny that the graco infant seat you want to buy her has the same weight limit.
As for being in a hot car, my son never complains about being put in it even in the summer, but my sons head and back still got sweaty where it had been touching the car seat (but not as bad as his snugride 22).
The fact that it has more space around the baby may help with it not being so hot, but well car's still get hot and babies still sweat. But I find infant seats just more convenient in general
Hope I helped with my ramblings, lol.