No, I don't think a 250F is totally out of the question, but for a first bike, it would be more of a challenge that a 150R due to the height/weight (not so much due to the power).
Ideally, if she could test ride a 150F and 250F, and see what she thinks before buying a bike, that would be the best idea. But if that's not an option, then another "test" could be to have her try to ride your 450F a little, just around the pits/parking lot/field/yard whatever, to see what she thinks about the taller and more heavy bike, and the general power. Don't worry about starting it, the 250F is MUCH easier to start than a 450. I can't start my husband's CRF450 to save my life, but my CRF250 starts super easy, usually 1-2 kicks. But the 450 height is similar to a 250F, and if she can handle the 450 height/weight with turning the bike in the pits/parking lot, then the 20 lb. lighter 250F should feel even better. Same with if she is okay with the bike height/weight with getting on and off the bike, coming to a complete stop when riding it, pushing it around (like when it's not running), etc.
I am 5'5", and am very comfortable on a 250F. But I did spend a year on an 80, and a few years on the taller 125 before switching to 250F's when they came out. (And I think the 250F power makes it sooo much easier to ride compared to a 125 for the average rider). To make my CRF250 feel "lower", I did shave the seat foam, raised the forks a little in the triple clamp, have the sag set correctly, and have taller pegs and lower bars. So the bike sits a little lower and feels lower, and I can touch my toes on both sides. And I personally couldn't ride a 150R in MX because after several years on a big bike, it is too small for me. I don't like the feeling of the smaller/lighter bike under me while jumping, and I don't like standing way over the bike on straightaways, but the 150 is awesome in the turns for me compared to a 250F. But if I raced more hare scrambles in the woods, I would love to have a CRF150R Expert.
Back to MX... just for an example, we had 10 girls at a local MX race yesterday.... 2 were on CRF150's, and 8 were on 250F's (well, maybe 1 or 2 were on 450's actually). The 150 riders were maybe 4'11" and 5'4", and have both been riding for at least 2-3 years. The 250F riders were from 5'0" - 5'9", and a 3-5 girls have raced for a year or less. A couple girls started on 250F's for their first bike, but most started on an 85/105 MX bike or a trail type of bike (CRF150/230/250X), but then quickly "outgrew" the smaller bike, and moved up to a 250F.
If you do decide to get the 150R for now, I think it would be a great choice for a first bike. But if your wife takes well to riding MX, I think she will likely want a 250F in the near future. Good luck! :ride: