i would tell her to get the convertable infant/toddler seat. he can still ride rear facing if he is supposed to be (my daughter outgrew her infant seat at 10 months but they are supposed to rear face til 1 yr) and then when he gets big enough to ride forward can be turned around with out having to buy a new seat
A convertible that fits both the baby and in her vehicle. Seriously...if both don't fit, it's not safe. I know I can get any carseat in my vehicle (I drive a Suburban) but I also know my one carseat won't fit in MIL's car correctly.